feat(platform): add Bots settings page with per-platform link management (#13222)

## Why

Users had no way to see which chat-bot platforms they'd connected
AutoGPT to, what their DM link looked like, or which servers their bots
were linked to. The existing `/settings/integrations` covers third-party
API integrations (Gmail, GitHub, etc.) but doesn't surface chat-bot
connections, which behave differently — they're per-deployment, can be
unlinked, and have an "add bot to server" entry point.

## What

A new **Bots** tab under Settings that dynamically lists every bot
platform enabled on this deployment (Discord today, more later as their
adapters land). For each platform: an "Add bot to X" button, a DM tile
showing your linked DM channel, and a list of linked servers each with
its own Unlink action.

## How

- `GET /api/platform-linking/platforms` returns one `BotPlatformInfo`
per enabled platform + caller's DM/server links. Platforms whose adapter
isn't configured are omitted, so future platforms stay hidden until
they're wired up.
- A small `registry.py` is the source of truth for which platforms
exist. Adding a new platform = adding one `PlatformMeta` row.
- Discord invite URL is built from a new
`AUTOPILOT_BOT_DISCORD_CLIENT_ID` secret + a hardcoded sensible
permissions default (`377957124928`) that can be overridden via
`AUTOPILOT_BOT_DISCORD_PERMISSIONS`.
- Frontend: standard `src/app/(platform)/settings/bots/` layout (page +
hooks + sub-components, render functions under 50 lines). Orval
generates the React Query hook.
- The server list distinguishes "server name unknown" (bot can't see the
server) with an amber indicator + tooltip, rather than rendering a raw
ID as if it were the name. Relies on the bot's name-refresh events from
the stacked PR underneath.

> ⚠️ **Stacked on #13221** (the bot-side name-refresh path). Merge that
first or servers the bot can see will appear as IDs until the bot
reconnects.

## Test plan

- [x] Backend: `poetry run pytest backend/api/features/platform_linking/
backend/platform_linking/ backend/copilot/bot/` — 217 pass
- [x] Frontend: `pnpm exec vitest run
src/app/\(platform\)/settings/bots` — 8 pass
- [x] Frontend: `pnpm format && pnpm exec next lint && pnpm exec
prettier --check src/app/api/openapi.json`
- [x] Manual: visited `/settings/bots`, confirmed Add bot button, DM
tile, server list with both happy-path and "Bot not in server" indicator


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