Add MySQL documentation with production HA guidance

[Copilot]

Comprehensive documentation for MySQL database backend support, completing the MySQL epic (#240). Adds MySQL alongside SQLite and PostgreSQL across all user-facing documentation with emphasis on production deployment best practices.

Core Changes

Database Selection & Configuration

• database-selection.md: Added MySQL comparison table, connection string formats, TLS options, and cloud provider examples (Aurora, RDS, Cloud SQL, Azure)
• configuration.md: MySQL environment variables, connection pool settings, and build tag requirements (-tags mysql)
• README.md: MySQL in supported databases list and configuration examples

Getting Started

• quickstart.md: Build options with MySQL support and startup examples
• installation.md: Build-from-source instructions with database-specific tags

Deployment

• helm.md: External and in-cluster MySQL deployment configurations
• deployment/index.md: MySQL quick start examples for Kubernetes
• architecture.md: Updated database layer and deployment diagrams

Production HA Guidance

Added warnings across all deployment docs:

⚠️ For production HA, use external managed MySQL:

• Amazon Aurora MySQL / RDS
• Google Cloud SQL
• Azure Database for MySQL
• Self-managed Group Replication or InnoDB Cluster

In-cluster MySQL StatefulSet is hardcoded to replicas: 1 (dev/test only).

Applied to:

• deploy/helm/llm-proxy/README.md (MySQL and PostgreSQL sections)
• docs/database/database-selection.md
• docs/database/docker-compose-{mysql,postgres}.md
• docs/deployment/helm.md

Build Tag Documentation

# SQLite only (default)
go build ./cmd/proxy

# With MySQL
go build -tags mysql ./cmd/proxy

# With PostgreSQL
go build -tags postgres ./cmd/proxy

# All databases
go build -tags "mysql,postgres" ./cmd/proxy

Connection String Examples

# MySQL with TLS
DATABASE_URL=user:pass@tcp(host:3306)/db?parseTime=true&tls=true

# PostgreSQL with SSL
DATABASE_URL=postgres://user:pass@host:5432/db?sslmode=require

Files changed: 14 (README, CHANGELOG, 8 docs/, 2 deploy/, 2 internal/)
Dependencies: Completes after #247 (driver), #248 (migrations), #253 (compose), #255 (helm), #256 (tests)

Original prompt

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This section details on the original issue you should resolve

<issue_title>[MySQL] Documentation Updates</issue_title>
<issue_description>## Summary

Update all relevant documentation to include MySQL as a supported database backend, including configuration guides, deployment instructions, and architectural documentation.

Parent Epic

Part of #240 - Add MySQL as Optional Database Backend

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🆕 Additional Items from PR Review

Single-Replica StatefulSet HA Guidance

During the review of PR sofatutor/llm-proxy#255 (Helm Chart MySQL Support), an important documentation item was identified:

The in-cluster MySQL StatefulSet is hardcoded to replicas: 1. The documentation must include clear guidance that:

⚠️ For production high-availability (HA) deployments, use an external managed MySQL service:

• Amazon Aurora MySQL / Amazon RDS for MySQL
• Google Cloud SQL for MySQL
• Azure Database for MySQL
• Self-managed MySQL Group Replication or InnoDB Cluster

Files requiring this note:

• deploy/helm/llm-proxy/README.md - MySQL section
• docs/database/database-selection.md - MySQL comparison
• docs/deployment/kubernetes.md - MySQL deployment section

The in-cluster MySQL option should be clearly positioned as development/testing only.

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Documentation Structure

flowchart TB
    subgraph CoreDocs["Core Documentation"]
        README["README.md"]
        DOCSIDX["docs/README.md"]
    end
    
    subgraph GettingStarted["Getting Started"]
        CONFIG["configuration.md"]
        QUICK["quickstart.md"]
    end
    
    subgraph DatabaseDocs["Database Documentation"]
        DBSEL["database-selection.md"]
        DCMYSQL["docker-compose-mysql.md (NEW)"]
        DBREADME["internal/database/README.md"]
    end
    
    subgraph DeployDocs["Deployment Documentation"]
        DOCKER["deployment/docker.md"]
        K8S["deployment/kubernetes.md"]
        HELMRM["helm/README.md"]
    end
    
    subgraph ArchDocs["Architecture Documentation"]
        ARCHIDX["architecture/index.md"]
        ARCHDB["architecture/database.md"]
    end
    
    subgraph ConfigDocs["Configuration"]
        ENVEX[".env.example"]
        ENVDOC["environment-variables.md"]
    end
    
    CoreDocs --> GettingStarted
    GettingStarted --> DatabaseDocs
    DatabaseDocs --> DeployDocs
    DeployDocs --> ArchDocs
    ConfigDocs --> DatabaseDocs

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Detailed Task Breakdown

Phase 1: Core Documentation

Task	Description	Files
1.1	Add MySQL to supported databases list	README.md
1.2	Update feature matrix	README.md
1.3	Update quick start examples	README.md
1.4	Update documentation index	docs/README.md

Phase 2: Getting Started Guides

Task	Description	Files
2.1	Add MySQL configuration section	docs/getting-started/configuration.md
2.2	Add MySQL quick start option	docs/getting-started/quickstart.md
2.3	Document build tag requirements	docs/getting-started/configuration.md

Phase 3: Database Documentation

Task	Description	Files
3.1	Add MySQL to comparison table	docs/database/database-selection.md
3.2	Document MySQL pros/cons	docs/database/database-selection.md
3.3	Create MySQL Docker guide	docs/database/docker-compose-mysql.md
3.4	Update internal database README	internal/database/README.md
3.5	Document connection string format	Multiple files
3.6	Add HA guidance for production	Multiple files (see above)

Phase 4: Deployment Documentation

Task	Description	Files
4.1	Add MySQL Docker Compose section	docs/deployment/docker.md
4.2	Add MySQL Helm configuration	docs/deployment/kubernetes.md
4.3	Update Helm chart README	deploy/helm/llm-proxy/README.md
4.4	Document build arguments	docs/deployment/docker.md
4.5	Add in-cluster MySQL limitations note	deploy/helm/llm-proxy/README.md

Phase 5: Architecture Documentation

Task	Description	Files
5.1	Add MySQL to database layer	docs/architecture/index.md
5.2	Document MySQL integration	docs/architecture/database.md
5.3	Update architecture diagrams	Multiple files

Phase 6: Configuration Documentation

Task	Description	Files
6.1	Add MySQL environment variables	.env.example
6.2	Document MySQL variables	docs/configuration/environment-variables.md
6.3	Add example configurations	Multiple files

Phase 7: Changelog and Release Notes

Task	Description	Files
7.1	Add MySQL feature entry	CHANGELOG.md
7.2	Document ...

• Fixes [MySQL] Documentation Updates #246

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[Copilot]

Pull request overview

This PR adds comprehensive MySQL documentation to complete the MySQL epic (#240), integrating MySQL alongside SQLite and PostgreSQL across all user-facing documentation with a strong emphasis on production deployment best practices and high-availability guidance.

Key changes:

• Added MySQL to all database comparison tables and configuration guides with consistent connection string formats and build tag documentation
• Included production HA warnings across deployment documentation, clarifying that in-cluster MySQL/PostgreSQL StatefulSets are development/testing only
• Provided cloud provider examples (Aurora, RDS, Cloud SQL, Azure) for production MySQL deployments

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
CHANGELOG.md	Added entry documenting MySQL documentation updates
README.md	Updated database support section to include MySQL with connection pool settings and build requirements
docs/README.md	Added MySQL Docker Compose guide link and updated database selection guide description
docs/getting-started/quickstart.md	Added MySQL build options and startup examples with Docker Compose profile
docs/getting-started/installation.md	Added MySQL build instructions and startup commands with connection string examples
docs/getting-started/configuration.md	Added comprehensive MySQL configuration section with connection string format, TLS options, and version compatibility
docs/deployment/index.md	Added Helm quick start example for production MySQL deployment with external database
docs/deployment/helm.md	Added MySQL external deployment section with HA warning and build requirements
docs/database/docker-compose-postgres.md	Added production HA warning for PostgreSQL Docker Compose setup
docs/database/docker-compose-mysql.md	Added production HA warning for MySQL Docker Compose setup
docs/database/database-selection.md	Expanded comparison table to include MySQL, added comprehensive MySQL section with configuration, cloud examples, and production deployment guidance
docs/architecture/architecture.md	Updated database layer description and deployment diagrams to include MySQL support and build tags
deploy/helm/llm-proxy/README.md	Enhanced HA warnings for both PostgreSQL and MySQL in-cluster StatefulSets with specific managed service recommendations
internal/database/README.md	Updated architecture diagrams, comparison table, and file structure to include MySQL support