feat(mcp): OAuth so claude.ai and ChatGPT can connect as connectors

MindSift becomes the OAuth authorization server for its own MCP
endpoint: discovery (RFC 9728/8414), dynamic client registration,
authorization code with PKCE (S256), rotating refresh tokens with reuse
detection. The consent page lives in the app at /oauth/authorize. A
grant is an api_keys entry (kind 'oauth'), so scopes, origin badges,
renaming and revoking work as for keys; tokens are valid for /mcp only.
Unused connections expire after OAUTH_IDLE_DAYS (default 90). Requires
BASE_URL; off in demo mode. Release 0.51.0.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Doku/reverse-proxy.md

@@ -12,6 +12,7 @@ Dieses Dokument beschreibt die empfohlene Konfiguration von Reverse-Proxies und
 | **Server-Sent Events (SSE)** | 443 (HTTPS) / 80 (HTTP) | `X-Accel-Buffering: no`<br>`proxy_buffering off`<br>`proxy_cache off` | Fallback bei blockiertem WebSocket-Upgrade |
 | **REST API & Uploads** | 443 (HTTPS) / 80 (HTTP) | `client_max_body_size 64M` | Notizen, Import/Export (JSON) |
 | **Client-IP** | – | `X-Real-IP` / `X-Forwarded-For` | Login-Sperre nach Fehlversuchen trifft den richtigen Absender |
+| **OAuth (MCP)** | 443 (HTTPS) | `/.well-known/oauth-*` und `/oauth/*` an MindSift durchreichen (mit `location /` automatisch), `BASE_URL` gesetzt | claude.ai und ChatGPT als Connector |
 
 ### Client-IP und `TRUSTED_PROXIES`