feat: Platform bot linking API for multi-platform CoPilot

Adds the account linking system that enables CoPilot to work across
multiple chat platforms (Discord, Telegram, Slack, Teams, etc.) via
the Vercel Chat SDK.

## What this adds

### Database (Prisma + migration)
- PlatformType enum (DISCORD, TELEGRAM, SLACK, TEAMS, WHATSAPP, GITHUB, LINEAR)
- PlatformLink model - maps platform user IDs to AutoGPT accounts
  - Unique constraint on (platform, platformUserId)
  - One AutoGPT user can link multiple platforms
- PlatformLinkToken model - one-time tokens for the linking flow

### API endpoints (/api/platform-linking)

Bot-facing (called by the bot service):
- POST /tokens - Create a link token for an unlinked platform user
- GET /tokens/{token}/status - Check if linking is complete
- POST /resolve - Resolve platform identity → AutoGPT user ID

User-facing (JWT auth required):
- POST /tokens/{token}/confirm - Complete the link (user logs in first)
- GET /links - List all linked platform identities
- DELETE /links/{link_id} - Unlink a platform identity

## Linking flow
1. User messages bot on Discord/Telegram/etc
2. Bot calls POST /resolve → not linked
3. Bot calls POST /tokens → gets link URL
4. Bot sends user the link
5. User clicks → logs in to AutoGPT → frontend calls POST /confirm
6. Bot detects link on next message (or polls /status)