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

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

.env.example

@@ -159,6 +159,16 @@ SEARXNG_INSTANCE=http://localhost:8080
 # Example: 52428800 = 50 MB.
 # ODYSSEUS_CHAT_UPLOAD_MAX_BYTES=10485760
 
+# Other per-feature upload size caps in bytes. All are validated and optional;
+# defaults shown. An invalid value (non-integer or < 1) fails fast at startup.
+# ODYSSEUS_GALLERY_UPLOAD_MAX_BYTES=104857600            # gallery image upload (100 MB)
+# ODYSSEUS_GALLERY_TRANSFORM_UPLOAD_MAX_BYTES=26214400   # gallery transform input (25 MB)
+# ODYSSEUS_MEMORY_IMPORT_MAX_BYTES=10485760              # memory import file (10 MB)
+# ODYSSEUS_PERSONAL_UPLOAD_MAX_BYTES=26214400            # personal document upload (25 MB)
+# ODYSSEUS_EMAIL_COMPOSE_UPLOAD_MAX_BYTES=26214400       # email compose attachment (25 MB)
+# ODYSSEUS_STT_MAX_AUDIO_BYTES=26214400                  # speech-to-text audio (25 MB)
+# ODYSSEUS_ICS_MAX_BYTES=10485760                        # calendar .ics import (10 MB)
+
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