Fix: Use validation context for drafts instead of skipping validations

[bellisabell]

Problem

In DraftController#update, drafts were saved with save!(validate: false) which bypasses all model validations. This is dangerous because it allows any data to be saved to the database, including data that violates length constraints or other integrity checks.

Solution

Use Rails validation contexts to selectively skip only the presence validations when saving drafts:

1. Changed save!(validate: false) to save!(context: :draft)
2. Added draft_context? method to Post model
3. Made presence validations (subject presence, body_cannot_be_empty) conditional with unless: :draft_context?
4. Length validation still enforced - subject max 255 chars is always validated

Benefits

• Drafts can still be saved without subject/body (as before)
• Data integrity is preserved (length limits enforced)
• More maintainable than validate: false
• Follows Rails best practices for conditional validations

Closes #23