Implement message model

[Iiquidate]

Create apps/server/src/models/messageModel.ts with the database query functions for messages. This follows the same pattern as channelModel.ts.

Queries needed:

getMessagesByChannelId(channelId) - fetch messages for a given channel, ordered by created_at

createMessage(channelId, userId, content) — insert a new message

deleteMessage(messageId)
— delete a single message (for admin moderation)

purgeOldMessages(hours)
— delete messages older than a given threshold

[ShreyaRajaram]

Pushed code for this to features/messages-db-queries branch