feat(subscriptions): add subscription change management with upgrade/downgrade scheduling

- Add subscription_change_pending_tier and subscription_change_pending_date fields to UserProfile
- Create migration 019_add_subscription_change_pending
- Add apply_pending_subscription_changes(), request_subscription_change(), cancel_pending_subscription_change() utilities
- Add POST /api/subscriptions/change and DELETE /api/subscriptions/change endpoints
- Update GET /api/subscriptions/my to apply pending changes and return pending change info
- Update subscription view to apply pending changes and pass period_start + pending info
- Update subscription.html: per-tier action buttons (upgrade/downgrade/cancel), pending-change banner, period start date
- Add Celery daily task apply_pending_subscription_changes_all at 00:05 UTC
- Add 19 new tests covering all new utility functions and API endpoints

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

app/celery_worker.py

@@ -25,6 +25,7 @@ from app.tasks.process_with_ocr import process_with_ocr  # noqa: F401
 from app.tasks.refine_text_with_gpt import refine_text_with_gpt  # noqa: F401
 from app.tasks.rotate_pdf_pages import rotate_pdf_pages  # noqa: F401
 from app.tasks.send_to_all import send_to_all_destinations  # noqa: F401
+from app.tasks.subscription_tasks import apply_pending_subscription_changes_all  # noqa: F401
 
 # Import new send tasks
 from app.tasks.upload_to_dropbox import upload_to_dropbox  # noqa: F401
@@ -104,6 +105,12 @@ celery.conf.beat_schedule = {
         "schedule": crontab(minute="*/5"),  # Every 5 minutes
         "options": {"expires": 240},  # 4 minutes expiry
     },
+    # Apply scheduled subscription downgrades daily at 00:05 UTC
+    "apply-pending-subscription-changes": {
+        "task": "app.tasks.subscription_tasks.apply_pending_subscription_changes_all",
+        "schedule": crontab(hour="0", minute="5"),  # 00:05 UTC daily
+        "options": {"expires": 3600},
+    },
 }
 
 # Remove None entries from beat_schedule