Feature/celery notifications

[GitGuru-sudo]

Summary

Move notification creation off the request thread onto a Celery worker backed by the already-configured Redis broker. Adds the Celery app, a tasks package with send_notification_task (own DB session, retries, self-guard), and a NotificationService.enqueue helper with synchronous fallback when the broker is down. Routers now enqueue instead of writing inline.

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Related Issue

Closes #161 (ECSoC 2026)

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Type of Change

• Bug fix
• New feature
• Documentation update
• Performance improvement
• Refactoring
• UI/UX enhancement
• Tests
• Other

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Changes Made

• Create backend/app/core/celery_app.py — Celery app with JSON serializer, timeouts, settings-driven eager flag
• Edit backend/app/core/config.py — add CELERY_TASK_ALWAYS_EAGER toggle
• Create backend/app/tasks/__init__.py — exports send_notification_task
• Create backend/app/tasks/notification_tasks.py — task with own DB session, 3 retries, reuses PR 1's notify()
• Add enqueue() helper to NotificationService — builds JSON-safe payload, calls .delay(), falls back to sync notify() on broker failure
• Switch all PR 1 call sites from notify( → enqueue( in followers.py, messages.py, applications.py
• Add worker service to docker-compose.yml (depends on postgres + redis)
• Add CELERY_TASK_ALWAYS_EAGER to .env.example
• Create backend/tests/test_notification_tasks.py — 3 tests (eager mode)

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Screenshots (if applicable)

N/A — backend-only change.

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Testing

• Tested locally
• Existing tests pass
• Added new tests
• UI verified
• Cross-browser tested (if applicable)

Additional testing notes: Tasks run eagerly (CELERY_TASK_ALWAYS_EAGER=True) in tests — no broker needed. Three tests cover: task creates a notification in DB, task skips self-notification (returns None), and full router→enqueue→task integration via TestClient. Verified via pytest on SQLite.

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Checklist

• My code follows the project's coding standards.
• I have tested my changes locally.
• I have updated the documentation where necessary.
• My changes do not introduce new warnings or errors.
• I have reviewed my own code.
• This pull request focuses on a single feature or fix.
• I have linked the related issue.

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Additional Notes

PR 2 of 2. Depends on PR 1 (feature/wire-notifications — auto-trigger notifications). The synchronous notify() is deliberately kept as the single insert path reused by the task — no duplicated logic.

Architecture:
POST /followers/{id} Celery worker (separate process)
send_notification_task db = SessionLocal()
.delay(payload) ───────► NotificationService.notify(db, ...)
return follow (instant) db.commit(); db.close()

[nensii21]

add screenshots and videos showing what changes have been made and also your CI jobs are failing please update it.

[GitGuru-sudo]

@nensii21 sure i look into that

[GitGuru-sudo]

@nensii21 as is a a backened feature so i cannot give u a screenshort for this