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

src/upload_limits.py

@@ -33,6 +33,34 @@ def get_chat_upload_max_bytes() -> int:
     return read_byte_limit_env(CHAT_UPLOAD_MAX_BYTES_ENV, DEFAULT_CHAT_UPLOAD_MAX_BYTES)
 
 
+# Per-route upload byte-limits, single-sourced here (issue #3364). Each is
+# validated + env-overridable via read_byte_limit_env: set the matching
+# ODYSSEUS_*_MAX_BYTES env var to an integer byte count to tune it; an invalid
+# value fails fast at import rather than crashing mid-request. Defaults match
+# the prior per-route values, so behavior is unchanged unless an env var is set.
+GALLERY_UPLOAD_MAX_BYTES = read_byte_limit_env(
+    "ODYSSEUS_GALLERY_UPLOAD_MAX_BYTES", 100 * 1024 * 1024
+)
+GALLERY_TRANSFORM_UPLOAD_MAX_BYTES = read_byte_limit_env(
+    "ODYSSEUS_GALLERY_TRANSFORM_UPLOAD_MAX_BYTES", 25 * 1024 * 1024
+)
+MEMORY_IMPORT_MAX_BYTES = read_byte_limit_env(
+    "ODYSSEUS_MEMORY_IMPORT_MAX_BYTES", 10 * 1024 * 1024
+)
+PERSONAL_UPLOAD_MAX_BYTES = read_byte_limit_env(
+    "ODYSSEUS_PERSONAL_UPLOAD_MAX_BYTES", 25 * 1024 * 1024
+)
+EMAIL_COMPOSE_UPLOAD_MAX_BYTES = read_byte_limit_env(
+    "ODYSSEUS_EMAIL_COMPOSE_UPLOAD_MAX_BYTES", 25 * 1024 * 1024
+)
+STT_MAX_AUDIO_BYTES = read_byte_limit_env(
+    "ODYSSEUS_STT_MAX_AUDIO_BYTES", 25 * 1024 * 1024
+)
+ICS_MAX_BYTES = read_byte_limit_env(
+    "ODYSSEUS_ICS_MAX_BYTES", 10 * 1024 * 1024
+)
+
+
 async def read_upload_limited(upload: UploadFile, limit: int, label: str = "Upload") -> bytes:
     """Read an UploadFile with a hard byte cap."""
     data = await upload.read(limit + 1)