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/tasks/subscription_tasks.py

@@ -0,0 +1,50 @@
+"""Celery task to apply pending subscription changes that have become due.
+
+Runs daily to ensure that scheduled downgrades are applied on time.
+"""
+
+import logging
+
+from app.celery_app import celery
+
+logger = logging.getLogger(__name__)
+
+
+@celery.task(name="app.tasks.subscription_tasks.apply_pending_subscription_changes_all")
+def apply_pending_subscription_changes_all() -> dict[str, int]:
+    """Apply all pending subscription changes whose effective date has arrived.
+
+    Iterates over every ``UserProfile`` that has a pending change and calls
+    :func:`app.utils.subscription.apply_pending_subscription_changes` for
+    each one.
+
+    Returns:
+        A dict with ``{"applied": <count>, "checked": <count>}``.
+    """
+    from app.database import SessionLocal
+    from app.models import UserProfile
+    from app.utils.subscription import apply_pending_subscription_changes
+
+    db = SessionLocal()
+    applied = 0
+    checked = 0
+    try:
+        profiles = (
+            db.query(UserProfile)
+            .filter(
+                UserProfile.subscription_change_pending_tier.isnot(None),
+                UserProfile.subscription_change_pending_date.isnot(None),
+            )
+            .all()
+        )
+        for profile in profiles:
+            checked += 1
+            if apply_pending_subscription_changes(db, profile.user_id):
+                applied += 1
+    except Exception as exc:
+        logger.error("Error in apply_pending_subscription_changes_all: %s", exc)
+    finally:
+        db.close()
+
+    logger.info("apply_pending_subscription_changes_all: checked=%d applied=%d", checked, applied)
+    return {"checked": checked, "applied": applied}