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.py

@@ -10,6 +10,15 @@ class ReactionSummary(BaseModel):
     user_ids: list[str]
 
 
+class MessageFileInfo(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+
+    id: uuid.UUID
+    filename: str
+    size_bytes: int
+    content_type: str
+
+
 class MessageRead(BaseModel):
     model_config = ConfigDict(from_attributes=True)
 
@@ -19,6 +28,7 @@ class MessageRead(BaseModel):
     username: str
     content: str | None
     image_id: uuid.UUID | None
+    file: MessageFileInfo | None
     reactions: list[ReactionSummary]
     created_at: datetime
     edited_at: datetime | None