fix(backend): require Discord server managers for bot setup (#13092)

### Why / What / How

Why: Discord `/setup` links an entire server to an AutoGPT account, so
it should only be available to users trusted to manage that server.
Without this guard, any server member could initiate the link flow for
the whole Discord server.

What: Require Discord `Manage Server` permission for the AutoPilot bot
`/setup` command and add coverage for the permission check. Also keeps
the message ignore behavior explicit: the bot ignores its own messages
while still allowing other bots to participate normally.

How: Adds Discord command default permissions via
`app_commands.default_permissions(manage_guild=True)` and a runtime
permission guard in the setup handler before creating link tokens.
Extracts the self-message check into a small adapter helper with tests.

### Changes 🏗️

- Require `Manage Server` permission for Discord `/setup` command
registration.
- Add a runtime `/setup` permission check that returns an ephemeral
error before token creation.
- Add tests covering non-admin setup rejection.
- Make Discord self-message ignore behavior explicit and test that other
bot/human messages are still accepted.

### Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
- [x] `cd backend && poetry run pytest backend/copilot/bot
backend/platform_linking backend/api/features/platform_linking -q`
- [x] `cd backend && poetry run pyright
backend/copilot/bot/adapters/discord backend/util/settings.py`
  - [x] `cd backend && poetry run lint --skip-pyright`

#### For configuration changes:

- [x] `.env.default` is updated or already compatible with my changes
- [x] `docker-compose.yml` is updated or already compatible with my
changes
- [x] I have included a list of my configuration changes in the PR
description (under **Changes**)

No configuration changes required.