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/core/celery_config.py

@@ -8,8 +8,17 @@ It includes:
 - Celery Beat schedule for periodic tasks
 - Task retry policies
 - Sentry integration for error tracking
+- Module-based task discovery (discovers tasks from app/modules/*/tasks/)
+
+Task Discovery:
+- Legacy tasks: Explicitly listed in the 'include' parameter
+- Module tasks: Auto-discovered via discover_module_tasks()
+
+As modules are migrated, their tasks will move from the legacy include list
+to automatic discovery from the module's tasks/ directory.
 """
 
+import logging
 import os
 
 import sentry_sdk
@@ -17,6 +26,8 @@ from celery import Celery
 from celery.schedules import crontab
 from sentry_sdk.integrations.celery import CeleryIntegration
 
+logger = logging.getLogger(__name__)
+
 # Redis URL from environment or default
 REDIS_URL = os.getenv("REDIS_URL", "redis://localhost:6379/0")
 
@@ -34,19 +45,49 @@ if SENTRY_DSN:
         send_default_pii=True,
     )
 
+# =============================================================================
+# TASK DISCOVERY
+# =============================================================================
+# Legacy tasks (will be migrated to modules over time)
+LEGACY_TASK_MODULES = [
+    "app.tasks.celery_tasks.marketplace",
+    "app.tasks.celery_tasks.letzshop",
+    "app.tasks.celery_tasks.subscription",
+    "app.tasks.celery_tasks.export",
+    "app.tasks.celery_tasks.code_quality",
+    "app.tasks.celery_tasks.test_runner",
+]
+
+
+def get_all_task_modules() -> list[str]:
+    """
+    Get all task modules (legacy + module-based).
+
+    Returns:
+        Combined list of legacy task modules and discovered module tasks
+    """
+    all_modules = list(LEGACY_TASK_MODULES)
+
+    try:
+        from app.modules.tasks import discover_module_tasks
+
+        module_tasks = discover_module_tasks()
+        all_modules.extend(module_tasks)
+        logger.info(f"Discovered {len(module_tasks)} module task packages")
+    except ImportError as e:
+        logger.warning(f"Could not import module task discovery: {e}")
+    except Exception as e:
+        logger.error(f"Error discovering module tasks: {e}")
+
+    return all_modules
+
+
 # Create Celery application
 celery_app = Celery(
     "wizamart",
     broker=REDIS_URL,
     backend=REDIS_URL,
-    include=[
-        "app.tasks.celery_tasks.marketplace",
-        "app.tasks.celery_tasks.letzshop",
-        "app.tasks.celery_tasks.subscription",
-        "app.tasks.celery_tasks.export",
-        "app.tasks.celery_tasks.code_quality",
-        "app.tasks.celery_tasks.test_runner",
-    ],
+    include=get_all_task_modules(),
 )
 
 # =============================================================================
@@ -98,7 +139,9 @@ celery_app.conf.task_routes = {
 # =============================================================================
 # CELERY BEAT SCHEDULE - Periodic tasks
 # =============================================================================
-celery_app.conf.beat_schedule = {
+
+# Legacy scheduled tasks (will be migrated to module definitions)
+LEGACY_BEAT_SCHEDULE = {
     # Reset usage counters at start of each period
     "reset-period-counters-daily": {
         "task": "app.tasks.celery_tasks.subscription.reset_period_counters",
@@ -131,6 +174,33 @@ celery_app.conf.beat_schedule = {
     },
 }
 
+
+def get_full_beat_schedule() -> dict:
+    """
+    Get complete beat schedule (legacy + module-defined).
+
+    Returns:
+        Merged beat schedule from legacy tasks and module definitions
+    """
+    schedule = dict(LEGACY_BEAT_SCHEDULE)
+
+    try:
+        from app.modules.tasks import build_beat_schedule
+
+        module_schedule = build_beat_schedule()
+        schedule.update(module_schedule)
+        if module_schedule:
+            logger.info(f"Added {len(module_schedule)} scheduled tasks from modules")
+    except ImportError as e:
+        logger.warning(f"Could not import module beat schedule builder: {e}")
+    except Exception as e:
+        logger.error(f"Error building module beat schedule: {e}")
+
+    return schedule
+
+
+celery_app.conf.beat_schedule = get_full_beat_schedule()
+
 # =============================================================================
 # QUEUE CONFIGURATION
 # =============================================================================