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>

docs/proposals/module-migration-plan.md

@@ -29,7 +29,7 @@ Transform the platform from a monolithic structure to self-contained modules whe
 |--------|---------------|---------------|----------|--------|-------|--------|
 | `cms` | Core | ✅ **Complete** | ✅ | ✅ | - | Done |
 | `payments` | Optional | 🟡 Partial | ✅ | ✅ | - | Done |
-| `billing` | Optional | 🔴 Shell | ❌ | ❌ | ❌ | Full |
+| `billing` | Optional | ✅ **Complete** | ✅ | ✅ | ✅ | Done |
 | `marketplace` | Optional | 🔴 Shell | ❌ | ❌ | ❌ | Full |
 | `orders` | Optional | 🔴 Shell | ❌ | ❌ | - | Full |
 | `inventory` | Optional | 🔴 Shell | ❌ | ❌ | - | Full |
@@ -110,85 +110,22 @@ app/tasks/celery_tasks/          # → Move to respective modules
 - Observability docs
 - Creating modules developer guide
 
----
+### ✅ Phase 4: Celery Task Infrastructure
+- Added `ScheduledTask` dataclass to `ModuleDefinition`
+- Added `tasks_path` and `scheduled_tasks` fields
+- Created `app/modules/task_base.py` with `ModuleTask` base class
+- Created `app/modules/tasks.py` with discovery utilities
+- Updated `celery_config.py` to use module discovery
 
-## Infrastructure Work (Remaining)
-
-### Phase 4: Celery Task Infrastructure
-
-Add task support to module system before migrating modules.
-
-#### 4.1 Update ModuleDefinition
-
-```python
-# app/modules/base.py - Add fields
-
-@dataclass
-class ScheduledTask:
-    """Celery Beat scheduled task definition."""
-    name: str                    # e.g., "billing.reset_counters"
-    task: str                    # Full task path
-    schedule: str | dict         # Cron string or crontab dict
-    args: tuple = ()
-    kwargs: dict = field(default_factory=dict)
-
-@dataclass
-class ModuleDefinition:
-    # ... existing fields ...
-
-    # Task configuration (NEW)
-    tasks_path: str | None = None
-    scheduled_tasks: list[ScheduledTask] = field(default_factory=list)
-```
-
-#### 4.2 Create Task Discovery
-
-```python
-# app/modules/tasks.py (NEW)
-
-def discover_module_tasks() -> list[str]:
-    """Discover task modules from all registered modules."""
-    ...
-
-def build_beat_schedule() -> dict:
-    """Build Celery Beat schedule from module definitions."""
-    ...
-```
-
-#### 4.3 Create Module Task Base
-
-```python
-# app/modules/task_base.py (NEW)
-
-class ModuleTask(Task):
-    """Base Celery task with DB session management."""
-
-    @contextmanager
-    def get_db(self):
-        ...
-```
-
-#### 4.4 Update Celery Config
-
-```python
-# app/core/celery_config.py
-
-from app.modules.tasks import discover_module_tasks, build_beat_schedule
-
-# Auto-discover from modules
-celery_app.autodiscover_tasks(discover_module_tasks())
-
-# Build schedule from modules
-celery_app.conf.beat_schedule = build_beat_schedule()
-```
-
-**Files to create:**
-- `app/modules/tasks.py`
-- `app/modules/task_base.py`
-
-**Files to modify:**
-- `app/modules/base.py`
-- `app/core/celery_config.py`
+### ✅ Phase 5: Billing Module Migration
+- Created `app/modules/billing/services/` with subscription, stripe, admin services
+- Created `app/modules/billing/models/` re-exporting from central location
+- Created `app/modules/billing/schemas/` re-exporting from central location
+- Created `app/modules/billing/tasks/` with 4 scheduled tasks
+- Created `app/modules/billing/exceptions.py`
+- Updated `definition.py` with self-contained configuration
+- Created backward-compatible re-exports in `app/services/`
+- Updated legacy celery_config.py to not duplicate scheduled tasks
 
 ---