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/modules/billing/definition.py

@@ -3,10 +3,10 @@
 Billing module definition.
 
 Defines the billing module including its features, menu items,
-and route configurations.
+route configurations, and scheduled tasks.
 """
 
-from app.modules.base import ModuleDefinition
+from app.modules.base import ModuleDefinition, ScheduledTask
 from models.database.admin_menu_config import FrontendType
 
 
@@ -54,6 +54,44 @@ billing_module = ModuleDefinition(
         ],
     },
     is_core=False,  # Billing can be disabled (e.g., internal platforms)
+    # =========================================================================
+    # Self-Contained Module Configuration
+    # =========================================================================
+    is_self_contained=True,
+    services_path="app.modules.billing.services",
+    models_path="app.modules.billing.models",
+    schemas_path="app.modules.billing.schemas",
+    exceptions_path="app.modules.billing.exceptions",
+    tasks_path="app.modules.billing.tasks",
+    # =========================================================================
+    # Scheduled Tasks
+    # =========================================================================
+    scheduled_tasks=[
+        ScheduledTask(
+            name="billing.reset_period_counters",
+            task="app.modules.billing.tasks.subscription.reset_period_counters",
+            schedule="5 0 * * *",  # Daily at 00:05
+            options={"queue": "scheduled"},
+        ),
+        ScheduledTask(
+            name="billing.check_trial_expirations",
+            task="app.modules.billing.tasks.subscription.check_trial_expirations",
+            schedule="0 1 * * *",  # Daily at 01:00
+            options={"queue": "scheduled"},
+        ),
+        ScheduledTask(
+            name="billing.sync_stripe_status",
+            task="app.modules.billing.tasks.subscription.sync_stripe_status",
+            schedule="30 * * * *",  # Hourly at :30
+            options={"queue": "scheduled"},
+        ),
+        ScheduledTask(
+            name="billing.cleanup_stale_subscriptions",
+            task="app.modules.billing.tasks.subscription.cleanup_stale_subscriptions",
+            schedule="0 3 * * 0",  # Weekly on Sunday at 03:00
+            options={"queue": "scheduled"},
+        ),
+    ],
 )