Add GET /api/v1/messages/:id endpoint to retrieve quoted/replied messages
for preview fill and navigation purposes. The endpoint includes authorization
checks ensuring only conversation participants can access the message.
Additionally improve file upload handling for native clients (expo-file-system)
by accepting explicit displayName from frontend, with sanitization to prevent
path traversal and 255-character length limit. File extension detection now
prioritizes the real display name over the multipart header filename.
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