Alpha: Messages — Send message to a user (text/emoji/gif/sticker) and update preview

[coderabbitai]

Context / backlinks:

• Requested by @shiv7shukla on March 17, 2026
• PR added signupcontroller #1: added signupcontroller #1

Goal:
Allow sending a message to a peer and update the conversation preview.

Scope:

• Endpoint: POST /conversations/messages/:userId with typed content (text | emoji | gif | sticker)
• Create conversation if it does not exist yet
• Update conversation preview with a sensible fallback for non-text content

Acceptance criteria:

• Authenticated requester can POST message to another valid user
• Response returns the saved message and conversationId
• Conversation preview updates for all content types (not undefined)
• Conversation “last active” reflects the latest message time
• Input validation errors return 400