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/tasks/__init__.py

@@ -0,0 +1,26 @@
+# app/modules/billing/tasks/__init__.py
+"""
+Billing module Celery tasks.
+
+Scheduled tasks for:
+- Resetting period counters
+- Checking trial expirations
+- Syncing with Stripe
+- Cleaning up stale subscriptions
+
+Note: capture_capacity_snapshot moved to monitoring module.
+"""
+
+from app.modules.billing.tasks.subscription import (
+    reset_period_counters,
+    check_trial_expirations,
+    sync_stripe_status,
+    cleanup_stale_subscriptions,
+)
+
+__all__ = [
+    "reset_period_counters",
+    "check_trial_expirations",
+    "sync_stripe_status",
+    "cleanup_stale_subscriptions",
+]