Added on-chain Stellar group ownership and owner-validated room update endpoint

[EmeditWeb]

Close #39

PR: Implement On-Chain Group Ownership

Summary

This change ties group ownership to a Stellar wallet address and enforces that only the wallet owner can update group metadata.

What changed

• Persisted owner_wallet on rooms during group creation
• Added PATCH /api/rooms/[roomId] endpoint to update room metadata
• Validates update requests by:
  • requiring walletAddress and signature
  • verifying the signature against a one-time nonce
  • checking that the wallet address matches the room owner
• Included owner_wallet in on-chain metadata hash verification
• Added DB migration to add owner_wallet column to public.rooms

Testing

1. Apply migration: run scripts/012_add_room_owner_wallet.sql in Supabase
2. Create a group with a connected Stellar wallet
3. Confirm the new room record includes owner_wallet
4. Request a nonce via /api/auth/nonce, sign it with the owner wallet, then call PATCH /api/rooms/<roomId> with walletAddress, signature, and updated metadata
5. Confirm updates succeed only when the correct owner wallet and signature are provided, and fail otherwise

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[SudiptaPaul-31]

@EmeditWeb Resolve merge conflicts