feat: Chat-Archive integration on contact detail page

- Add proxy endpoint GET /api/contacts/{id}/messages (forwards to Chat-Archive API)
- Add chat_sender_name field to Account config for sender matching
- Add Infinite Scroll chat section with Messenger-style bubbles
- Umlaut-normalized name comparison (ae/oe/ue/ss)
- Add CHATAPI_ENABLED/URL/KEY environment variables
- Update README.md, SPEC.md, AGENTS.md

.env.example

@@ -29,3 +29,8 @@ API_PORT=8000
 
 # --- Web-URL (optional, wird in Geburtstags-Mails verlinkt) ---
 WEB_URL=https://kontakte.example.de
+
+# --- Chat-Archive API (optional, zeigt Chat-Nachrichten in der Kontakt-Detailseite) ---
+CHATAPI_ENABLED=false
+CHATAPI_URL=http://docker-host-pve.fritz.box:8420
+CHATAPI_KEY=change-me