feat: add Celery task infrastructure for module system

Phase 4 of module migration plan:
- Add ScheduledTask dataclass for declaring Celery Beat tasks
- Add tasks_path and scheduled_tasks fields to ModuleDefinition
- Create ModuleTask base class with database session management
- Create task discovery utilities (discover_module_tasks, build_beat_schedule)
- Update celery_config.py to discover and register module tasks
- Maintain backward compatibility with legacy task modules

Modules can now define tasks in their tasks/ directory and scheduled
tasks in their definition. The infrastructure supports gradual migration
of existing tasks from app/tasks/ to their respective modules.

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

app/modules/__init__.py

@@ -20,6 +20,7 @@ Module Hierarchy:
         └── Modules (Enabled features - Billing, Marketplace, Inventory, etc.)
             ├── Routes (API + Page routes)
             ├── Services (Business logic)
+            ├── Tasks (Celery background jobs)
             ├── Menu Items (Sidebar entries)
             ├── Templates (UI components)
             └── Migrations (Module-specific)
@@ -52,7 +53,14 @@ Usage:
         print(f"Module {data.module_code} enabled")
 """
 
-from app.modules.base import ModuleDefinition
+from app.modules.base import ModuleDefinition, ScheduledTask
+from app.modules.task_base import ModuleTask, DatabaseTask
+from app.modules.tasks import (
+    discover_module_tasks,
+    build_beat_schedule,
+    parse_schedule,
+    get_module_task_routes,
+)
 from app.modules.registry import (
     MODULES,
     CORE_MODULES,
@@ -76,6 +84,14 @@ from app.modules.events import (
 __all__ = [
     # Core types
     "ModuleDefinition",
+    "ScheduledTask",
+    # Task support
+    "ModuleTask",
+    "DatabaseTask",
+    "discover_module_tasks",
+    "build_beat_schedule",
+    "parse_schedule",
+    "get_module_task_routes",
     # Module dictionaries
     "MODULES",
     "CORE_MODULES",