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/models.py

@@ -238,6 +238,13 @@ class UserProfile(Base):
     subscription_period_start = Column(DateTime(timezone=True), nullable=True)
     allow_overage = Column(Boolean, nullable=False, default=False, server_default="0")
 
+    # Pending subscription change (added in migration 019)
+    # When a user requests a downgrade, the new tier is stored here and the
+    # change is applied on `subscription_change_pending_date`.  Upgrades are
+    # applied immediately and these fields are left NULL.
+    subscription_change_pending_tier = Column(String(50), nullable=True)
+    subscription_change_pending_date = Column(DateTime(timezone=True), nullable=True)
+
     # Onboarding tracking (added in migration 017)
     onboarding_completed = Column(Boolean, nullable=False, default=False, server_default="0")
     onboarding_completed_at = Column(DateTime(timezone=True), nullable=True)