Add feature-profile endpoint and env-driven optional router profiles

- Add ALWRITY_FEATURE_PROFILE env var (precedence over ALWRITY_ROUTER_PROFILE)
- Add OPTIONAL_MODULE_MATRIX defining 'all' and 'default' profiles
- Add get_feature_profile_status() to RouterManager
- Add GET /api/feature-profile/status endpoint in main.py and app.py
- Returns active profile and enabled optional modules

backend/app.py

@@ -290,6 +290,11 @@ async def router_status():
     )
     return status
 
+@app.get("/api/feature-profile/status")
+async def feature_profile_status():
+    """Get feature profile status and enabled modules."""
+    return router_manager.get_feature_profile_status()
+
 # Onboarding management endpoints
 @app.get("/api/onboarding/status")
 async def onboarding_status():