Add special handling for log_retention_days and log_max_records features
that are stored as separate Redis keys instead of in the features hash.
- Store log overrides in dedicated keys (meta:log:retention_days,
meta:log:max_records) for runtime access by log cleanup cron
- Include log override values in ListFeatures response
- Support clearing overrides by setting value to 0
- Add comprehensive tests for log override CRUD operations
Add FeatureHandler to manage lightweight runtime configuration toggles
stored in the Redis `meta:features` hash. This enables dynamic control
over system behavior (e.g., storage backends) via the admin API.
- Add `GET /admin/features` to list flags
- Add `PUT /admin/features` to update flags
- Update README with feature flag documentation
