feat: complete billing module migration (Phase 5)

Migrates billing module to self-contained structure:
- Create app/modules/billing/services/ with subscription, stripe, admin services
- Create app/modules/billing/models/ re-exporting from central location
- Create app/modules/billing/schemas/ re-exporting from central location
- Create app/modules/billing/tasks/ with 4 scheduled Celery tasks
- Create app/modules/billing/exceptions.py with module-specific exceptions
- Update definition.py with is_self_contained=True and scheduled_tasks

Celery task migration:
- reset_period_counters -> billing module
- check_trial_expirations -> billing module
- sync_stripe_status -> billing module
- cleanup_stale_subscriptions -> billing module
- capture_capacity_snapshot remains in legacy (will go to monitoring)

Backward compatibility:
- Create re-exports in app/services/ for subscription, stripe, admin services
- Old import paths continue to work
- Update celery_config.py to use module-defined schedules

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/core/celery_config.py

@@ -49,10 +49,12 @@ if SENTRY_DSN:
 # TASK DISCOVERY
 # =============================================================================
 # Legacy tasks (will be migrated to modules over time)
+# NOTE: Most subscription tasks have been migrated to app.modules.billing.tasks
+#       The subscription module is kept for capture_capacity_snapshot (will move to monitoring)
 LEGACY_TASK_MODULES = [
     "app.tasks.celery_tasks.marketplace",
     "app.tasks.celery_tasks.letzshop",
-    "app.tasks.celery_tasks.subscription",
+    "app.tasks.celery_tasks.subscription",  # Kept for capture_capacity_snapshot only
     "app.tasks.celery_tasks.export",
     "app.tasks.celery_tasks.code_quality",
     "app.tasks.celery_tasks.test_runner",
@@ -141,32 +143,9 @@ celery_app.conf.task_routes = {
 # =============================================================================
 
 # Legacy scheduled tasks (will be migrated to module definitions)
+# NOTE: Subscription tasks have been migrated to billing module (see definition.py)
 LEGACY_BEAT_SCHEDULE = {
-    # Reset usage counters at start of each period
-    "reset-period-counters-daily": {
-        "task": "app.tasks.celery_tasks.subscription.reset_period_counters",
-        "schedule": crontab(hour=0, minute=5),  # 00:05 daily
-        "options": {"queue": "scheduled"},
-    },
-    # Check for expiring trials and send notifications
-    "check-trial-expirations-daily": {
-        "task": "app.tasks.celery_tasks.subscription.check_trial_expirations",
-        "schedule": crontab(hour=1, minute=0),  # 01:00 daily
-        "options": {"queue": "scheduled"},
-    },
-    # Sync subscription status with Stripe
-    "sync-stripe-status-hourly": {
-        "task": "app.tasks.celery_tasks.subscription.sync_stripe_status",
-        "schedule": crontab(minute=30),  # Every hour at :30
-        "options": {"queue": "scheduled"},
-    },
-    # Clean up stale/orphaned subscriptions
-    "cleanup-stale-subscriptions-weekly": {
-        "task": "app.tasks.celery_tasks.subscription.cleanup_stale_subscriptions",
-        "schedule": crontab(hour=3, minute=0, day_of_week=0),  # Sunday 03:00
-        "options": {"queue": "scheduled"},
-    },
-    # Capture daily capacity snapshot for analytics
+    # Capacity snapshot - will be migrated to monitoring module
     "capture-capacity-snapshot-daily": {
         "task": "app.tasks.celery_tasks.subscription.capture_capacity_snapshot",
         "schedule": crontab(hour=0, minute=0),  # Midnight daily