Add profile-driven feature runtime utilities

- Add feature_registry.py with FeatureGroup definitions for core, podcast, youtube, content_planning
- Add feature_profiles.py to parse ALWRITY_FEATURE_TO_ENABLE env var
- Add feature_runtime.py with is_enabled(), get_enabled_routers() helpers
- Fix syntax error in __init__.py (duplicate OnboardingManager)

Enables feature toggles via ALWRITY_FEATURE_TO_ENABLE environment variable.

backend/alwrity_utils/feature_runtime.py

@@ -0,0 +1,71 @@
+"""Runtime helpers for profile-driven feature toggles."""
+
+from __future__ import annotations
+
+from functools import lru_cache
+from typing import Tuple
+
+from .feature_profiles import expand_profiles, parse_feature_profiles
+from .feature_registry import FEATURE_GROUPS
+
+
+@lru_cache(maxsize=1)
+def _runtime_state() -> dict[str, Tuple[str, ...]]:
+    profiles = parse_feature_profiles()
+    expanded = expand_profiles(profiles)
+    
+    routers = []
+    startup_hooks = []
+    optional_services = []
+    enabled_features = set(expanded.groups)
+    
+    for group in expanded.groups:
+        feature_group = FEATURE_GROUPS[group]
+        routers.extend(feature_group.routers)
+        startup_hooks.extend(feature_group.startup_hooks)
+        optional_services.extend(feature_group.optional_services)
+        enabled_features.update(feature_group.features)
+    
+    return {
+        "profiles": expanded.profiles,
+        "groups": expanded.groups,
+        "routers": tuple(dict.fromkeys(routers)),
+        "startup_hooks": tuple(dict.fromkeys(startup_hooks)),
+        "optional_services": tuple(dict.fromkeys(optional_services)),
+        "features": tuple(sorted(enabled_features)),
+    }
+
+
+def get_active_profiles() -> Tuple[str, ...]:
+    """Return validated active profile names."""
+    return _runtime_state()["profiles"]
+
+
+def get_enabled_groups() -> Tuple[str, ...]:
+    """Return resolved feature-group names."""
+    return _runtime_state()["groups"]
+
+
+def get_enabled_routers() -> Tuple[str, ...]:
+    """Return enabled router import targets in `module:attribute` format."""
+    return _runtime_state()["routers"]
+
+
+def get_enabled_startup_hooks() -> Tuple[str, ...]:
+    """Return enabled startup hook import targets in `module:attribute` format."""
+    return _runtime_state()["startup_hooks"]
+
+
+def get_enabled_optional_services() -> Tuple[str, ...]:
+    """Return enabled optional service import targets in `module:attribute` format."""
+    return _runtime_state()["optional_services"]
+
+
+def is_enabled(feature: str) -> bool:
+    """Return True when a feature/group name is enabled by active profiles."""
+    return feature.strip().lower() in _runtime_state()["features"]
+
+
+def reset_feature_runtime_cache() -> None:
+    """Clear runtime cache (useful for tests)."""
+    _runtime_state.cache_clear()