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/schemas/__init__.py

@@ -0,0 +1,56 @@
+# app/modules/billing/schemas/__init__.py
+"""
+Billing module Pydantic schemas.
+
+Re-exports subscription schemas from the central schemas location.
+Provides a module-local import path while maintaining backwards compatibility.
+
+Usage:
+    from app.modules.billing.schemas import (
+        SubscriptionCreate,
+        SubscriptionResponse,
+        TierInfo,
+    )
+"""
+
+from models.schema.subscription import (
+    # Tier schemas
+    TierFeatures,
+    TierLimits,
+    TierInfo,
+    # Subscription CRUD schemas
+    SubscriptionCreate,
+    SubscriptionUpdate,
+    SubscriptionResponse,
+    # Usage schemas
+    SubscriptionUsage,
+    UsageSummary,
+    SubscriptionStatusResponse,
+    # Limit check schemas
+    LimitCheckResult,
+    CanCreateOrderResponse,
+    CanAddProductResponse,
+    CanAddTeamMemberResponse,
+    FeatureCheckResponse,
+)
+
+__all__ = [
+    # Tier schemas
+    "TierFeatures",
+    "TierLimits",
+    "TierInfo",
+    # Subscription CRUD schemas
+    "SubscriptionCreate",
+    "SubscriptionUpdate",
+    "SubscriptionResponse",
+    # Usage schemas
+    "SubscriptionUsage",
+    "UsageSummary",
+    "SubscriptionStatusResponse",
+    # Limit check schemas
+    "LimitCheckResult",
+    "CanCreateOrderResponse",
+    "CanAddProductResponse",
+    "CanAddTeamMemberResponse",
+    "FeatureCheckResponse",
+]