Add response envelope middleware to standardize JSON responses as
`{code,data,message}` with consistent business codes across endpoints.
Update Swagger annotations and tests to reflect the new response shape.
BREAKING CHANGE: API responses are now wrapped in a response envelope; clients must read payloads from `data` and handle `code`/`message` fields.
Add tests for UpdateFeatures endpoint covering:
- Regular feature flag keys with various value types
- Mixed keys combining feature flags with log retention settings
- Redis storage verification for both scenarios
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
