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/personal_routes.py

@@ -11,11 +11,9 @@ from src.rag_singleton import get_rag_manager
 from src.auth_helpers import require_privilege, require_user
 from core.middleware import require_admin
 from src.upload_handler import secure_filename
+from src.upload_limits import PERSONAL_UPLOAD_MAX_BYTES
 
 UPLOADS_DIR = PERSONAL_UPLOADS_DIR
-MAX_PERSONAL_UPLOAD_BYTES = int(
-    os.getenv("ODYSSEUS_PERSONAL_UPLOAD_MAX_BYTES", str(25 * 1024 * 1024))
-)
 
 logger = logging.getLogger(__name__)
 
@@ -208,8 +206,8 @@ def setup_personal_routes(personal_docs_manager, rag_manager, rag_available):
         for upload in files:
             try:
                 file_path, stored_name, safe_name = _unique_personal_upload_path(upload_dir, upload.filename)
-                content_bytes = await upload.read(MAX_PERSONAL_UPLOAD_BYTES + 1)
-                if len(content_bytes) > MAX_PERSONAL_UPLOAD_BYTES:
+                content_bytes = await upload.read(PERSONAL_UPLOAD_MAX_BYTES + 1)
+                if len(content_bytes) > PERSONAL_UPLOAD_MAX_BYTES:
                     logger.warning(f"Rejected oversized personal upload: {upload.filename!r}")
                     total_failed += 1
                     continue