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.
- Create `DailyStat` model for immutable daily metrics including
request counts, tokens, latency, and top models.
- Implement `DailyStatsJob` to aggregate `log_records` from the previous
day, running daily at 00:05 UTC.
- Register database migrations and schedule the job in the server.
- Add `last7d` and `last30d` period support to stats handler.
