Log volume optimization diff summary

1. apps/backend/src/middleware/access-log.ts
   - Successful GET/HEAD responses (2xx/3xx) now use DEBUG.
   - Writes and all 4xx/5xx responses remain INFO.

2. apps/backend/src/utils/logger.ts
   - Appends rotate at LOG_MAX_FILE_SIZE_MB (default 20MB).
   - Keeps LOG_MAX_FILES (default 5) files per channel/date.
   - Rotated files are included in retention cleanup.

3. Configuration/docs/tests
   - Added LOG_MAX_FILE_SIZE_MB and LOG_MAX_FILES to env overrides,
     Compose files, examples, runtime types, backend README, and tests.