Add generic file attachments to chat messages

Messages can now carry an arbitrary file (MessageFile), parallel to the
existing MessageImage feature rather than a refactor of it. Files serve
with Content-Disposition: attachment to force a download and prevent an
uploaded HTML/SVG from executing same-origin. No content-type allowlist,
same 8MB cap as images for now (a separate size-limit redesign is tracked
as its own issue).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/schemas/message_file.py

@@ -0,0 +1,10 @@
+import uuid
+
+from pydantic import BaseModel
+
+
+class MessageFileCreated(BaseModel):
+    id: uuid.UUID
+    filename: str
+    size_bytes: int
+    content_type: str