refactor(uploads): centralize upload byte-limits in upload_limits.py (#3364) (#3518)

Move every per-route upload byte-limit into src/upload_limits.py as a
validated, env-overridable constant via read_byte_limit_env:

- Add GALLERY_UPLOAD_MAX_BYTES, GALLERY_TRANSFORM_UPLOAD_MAX_BYTES,
  MEMORY_IMPORT_MAX_BYTES, PERSONAL_UPLOAD_MAX_BYTES,
  EMAIL_COMPOSE_UPLOAD_MAX_BYTES, STT_MAX_AUDIO_BYTES, ICS_MAX_BYTES.
- Routes import their constant instead of defining it locally: replaces 4
  raw int(os.getenv(...)) and removes 3 hardcoded literals.
- The 3 previously-hardcoded limits (email compose, STT audio, calendar
  ICS) are now env-overridable with the same ODYSSEUS_*_MAX_BYTES naming.
- Defaults unchanged, so behavior is unchanged unless an env var is set;
  an invalid value now fails fast with a clear message instead of a bare
  int() ValueError.
- Document all env vars in .env.example and the README.

Fixes #3364

routes/stt_routes.py

@@ -4,12 +4,10 @@
 from fastapi import APIRouter, HTTPException, UploadFile, File
 import logging
 
-from src.upload_limits import read_upload_limited
+from src.upload_limits import read_upload_limited, STT_MAX_AUDIO_BYTES
 
 logger = logging.getLogger(__name__)
 
-STT_MAX_AUDIO_BYTES = 25 * 1024 * 1024
-
 
 def setup_stt_routes(stt_service):
     """Setup STT routes with the provided STT service"""