Paginate monthly template usage stats

[whabanks]

Summary | Résumé

This PR aims to improve performance of the "Monthly Template Usage" page in admin with optimizations to the underlying query. For the average service load times should be similar, the real gain is for services with high sending volume across a large number of templates. The JSON payload from the API is bound by the page size, and the browser won't choke trying to render thousands of table rows.

Added fetch_monthly_template_usage_for_service_paginated dao method that performs pagination at the query level, splitting the query into two and running the more expensive par on the templates on the current "page" only.

1. Get the page of template_ids, sorting them by count descending. We use a combination of offset and limit to construct the page.
2. Fetch the detailed template stats using that subset of templates rather than fetching everything all at once.

Related Issues | Cartes liées

• https://app.zenhub.com/workspaces/notify-planning-614b3ad91bc2030015ed22f5/issues/gh/cds-snc/notification-planning/3208

Test instructions | Instructions pour tester la modification

• CI is passing
• Release instructions test has been performed

Release Instructions | Instructions pour le déploiement

1. Check out this branch and with your local Admin on main to ensure backwards compatibility (not the associated admin PR)

• The dashboard loads without error
• The /template-usage page loads without error (See all templates used link on the dashboard)

Reviewer checklist | Liste de vérification du réviseur

• This PR does not break existing functionality.
• This PR does not violate GCNotify's privacy policies.
• This PR does not raise new security concerns. Refer to our GC Notify Risk Register document on our Google drive.
• This PR does not significantly alter performance.
• Additional required documentation resulting of these changes is covered (such as the README, setup instructions, a related ADR or the technical documentation).

⚠ If boxes cannot be checked off before merging the PR, they should be moved to the "Release Instructions" section with appropriate steps required to verify before release. For example, changes to celery code may require tests on staging to verify that performance has not been affected.

[Copilot]

Pull request overview

This PR introduces database-level pagination for the admin “Monthly Template Usage” endpoint to avoid loading usage stats for all templates at once, aiming to improve performance for large services.

Changes:

• Added a new DAO method fetch_monthly_template_usage_for_service_paginated that pages by unique templates using a two-step query (page template_ids first, then fetch monthly rows for those templates).
• Updated GET /service/<id>/notifications/templates_usage/monthly to return a paginated response shape when a page query param is provided.
• Added DAO- and REST-level tests covering pagination shape, ordering, offsets, and basic validation.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
app/dao/fact_notification_status_dao.py	Adds the new paginated DAO query for monthly template usage.
app/service/rest.py	Adds query-param parsing and paginated response mode to the monthly template usage endpoint.
tests/app/dao/test_fact_notification_status_dao.py	Adds unit tests for the new paginated DAO behavior.
tests/app/service/test_statistics_rest.py	Adds endpoint tests for paginated response shape, ordering, links, and invalid args.

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[andrewleith]

LGTM