Deprecate legacy feature flags, use ALWRITY_ENABLED_FEATURES only

- Remove fallback to ALWRITY_FEATURE_PROFILE, ALWRITY_ROUTER_PROFILE
- Primary env var is now ALWRITY_ENABLED_FEATURES (backend)
- Primary env var is REACT_APP_ENABLED_FEATURES (frontend)
- Add deprecation comments to all get_enabled_features() functions
- Update demoMode.ts with clear deprecation notes

Usage:
  ALWRITY_ENABLED_FEATURES=podcast      # Podcast only
  ALWRITY_ENABLED_FEATURES=all          # All features (default)

backend/alwrity_utils/router_manager.py

@@ -83,17 +83,16 @@ class RouterManager:
     
     @staticmethod
     def get_enabled_features() -> set:
-        """Get enabled features from environment variable.
+        """Get enabled features from ALWRITY_ENABLED_FEATURES env var.
         
-        ALWRITY_ENABLED_FEATURES can be:
+        Values:
         - "all" - enable all features (default)
-        - comma-separated list: "podcast,blog-writer,youtube"
+        - comma-separated: "podcast,blog-writer,youtube"
         - single feature: "podcast"
+        
+        DEPRECATED: ALWRITY_FEATURE_PROFILE, ALWRITY_ROUTER_PROFILE, ALWRITY_FEATURE_TO_ENABLE
         """
-        env_value = os.getenv(
-            "ALWRITY_ENABLED_FEATURES",
-            os.getenv("ALWRITY_FEATURE_PROFILE", os.getenv("ALWRITY_ROUTER_PROFILE", "all"))
-        ).strip().lower()
+        env_value = os.getenv("ALWRITY_ENABLED_FEATURES", "all").strip().lower()
         
         if not env_value or env_value == "all":
             return {"all"}