Add channel controller and routes

[Iiquidate]

Create the first controller layer in the backend to expose channelModel
functions over HTTP. This establishes the controller/route pattern that
messages (and future resources) will follow, so structure decisions made here
will be reused.

Files to create:

• apps/server/src/controllers/channelController.ts: HTTP handlers that call
  channelModel functions. No raw SQL.
• apps/server/src/routes/channels.ts: wires verbs/paths/middleware to
  controller functions.
• Register the router in apps/server/src/index.ts under /channels.

Endpoints:

• GET /channels/official -> list official channels
• GET /channels/nearby?lat=&lng= -> list nearby community channels (validate
  lat/lng are numbers)
• GET /channels/:id -> fetch one channel, 404 if not found
• POST /channels (requireAuth) -> create a community channel; createdBy comes
  from req.user
• DELETE /channels/:id (requireAuth) -> delete a channel

Models already exist in
apps/server/src/models/channelModel.ts.

If you already started working on this don't worry too much about following the exact structure I outlined. Implement however you wish