chore: remove Sentry integration

Remove @sentry/nextjs and all related files since it was never actively used.
- Delete sentry.server.config.ts, sentry.edge.config.ts
- Delete sentry-example-page and sentry-example-api routes
- Clean up instrumentation.ts, global-error.tsx, middleware.ts
- Remove Sentry env vars from env.example and docs
- Update CLAUDE.md, copilot-instructions.md, PRODUCTION_READINESS.md

Middleware bundle reduced from 86KB to 34.8KB (-51KB).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.github/copilot-instructions.md

@@ -58,7 +58,7 @@ npm run db:seed      # Seed database
 - **CMS**: Directus (self-hosted, GraphQL, optional)
 - **Automation**: n8n webhooks (status, chat, hardcover, image generation)
 - **i18n**: next-intl (EN + DE)
-- **Monitoring**: Sentry
+- **Monitoring**: Console error logging (development mode only)
 - **Deployment**: Docker (standalone mode) + Nginx
 
 ### Key Directories
@@ -200,7 +200,6 @@ REDIS_URL=redis://...
 
 ### Optional
 ```bash
-SENTRY_DSN=...
 NEXT_PUBLIC_BASE_URL=https://dk0.dev
 ```