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/email_routes.py

@@ -35,7 +35,7 @@ from fastapi.responses import FileResponse
 from src.constants import DATA_DIR
 
 from src.llm_core import llm_call_async
-from src.upload_limits import read_upload_limited
+from src.upload_limits import read_upload_limited, EMAIL_COMPOSE_UPLOAD_MAX_BYTES
 
 from routes.email_helpers import (
     _strip_think, _extract_reply, _apply_email_style_mechanics, require_owner, require_user, _assert_owns_account,
@@ -58,7 +58,6 @@ from routes.email_pollers import _start_poller
 logger = logging.getLogger(__name__)
 
 ODYSSEUS_MAIL_ORIGIN = "odysseus-ui"
-EMAIL_COMPOSE_UPLOAD_MAX_BYTES = 25 * 1024 * 1024
 
 
 def _email_tag_owner_aliases(account_id: str | None, owner: str = "") -> list[str]: