fix(upload): configure chat attachment size limit (#2439)

src/upload_limits.py

@@ -1,7 +1,12 @@
 """Small helpers for route-local upload size caps."""
 
+import os
+
 from fastapi import HTTPException, UploadFile
 
+DEFAULT_CHAT_UPLOAD_MAX_BYTES = 10 * 1024 * 1024
+CHAT_UPLOAD_MAX_BYTES_ENV = "ODYSSEUS_CHAT_UPLOAD_MAX_BYTES"
+
 
 def format_byte_limit(limit: int) -> str:
     if limit % (1024 * 1024) == 0:
@@ -11,6 +16,23 @@ def format_byte_limit(limit: int) -> str:
     return f"{limit} bytes"
 
 
+def read_byte_limit_env(name: str, default: int) -> int:
+    raw = os.getenv(name)
+    if raw is None or not raw.strip():
+        return default
+    try:
+        limit = int(raw)
+    except ValueError as exc:
+        raise ValueError(f"{name} must be an integer byte count") from exc
+    if limit < 1:
+        raise ValueError(f"{name} must be greater than 0")
+    return limit
+
+
+def get_chat_upload_max_bytes() -> int:
+    return read_byte_limit_env(CHAT_UPLOAD_MAX_BYTES_ENV, DEFAULT_CHAT_UPLOAD_MAX_BYTES)
+
+
 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)