Add database-backed configuration (AppSetting model, ConfigService, settings API)

- Add AppSetting model for key-value settings storage in PostgreSQL
- Create ConfigService with DB-first, env-var-fallback resolution
- Add admin-only /api/v1/settings CRUD endpoints
- Update tasks.py to use ConfigService for SMTP config
- Seed default settings on application startup
- Add 24 unit tests for ConfigService (all passing)

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>
Agent-Logs-Url: https://github.com/christianlouis/pop_puller_to_gmail/sessions/ce88d4a8-d8c2-49b3-95a1-30592105a769

backend/app/api/v1/api.py

@@ -12,6 +12,7 @@ from app.api.v1.endpoints import (
     subscriptions,
     admin,
     providers,
+    app_settings,
 )
 
 api_router = APIRouter()
@@ -32,3 +33,4 @@ api_router.include_router(
     subscriptions.router, prefix="/subscriptions", tags=["Subscriptions"]
 )
 api_router.include_router(admin.router, prefix="/admin", tags=["Admin"])
+api_router.include_router(app_settings.router, prefix="/settings", tags=["Settings"])