Refactor to flat APIKey model with quota, admin UI, .env config, and Berlin timezone

- Remove User/Quota models; quota fields now live directly on APIKey
- Admin UI: login, API key management, settings (Ollama URL/model), proxy info display
- .env/.env.example: ADMIN_PASSWORD, PROXY_HOST/PORT, DATABASE_URL, APP_TZ
- Admin API runs on 127.0.0.1 only; proxy host/port configurable
- API keys support optional expires_at; verified against Europe/Berlin timezone
- Daily/monthly quota resets use Europe/Berlin midnight boundary
- Fix all tests to use new flat model; add expiry tests

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.env.example

@@ -0,0 +1,18 @@
+# Admin-Passwort für die Weboberfläche
+ADMIN_PASSWORD=change-me
+
+# Lokaler Endpunkt des Proxys (Admin-API bindet immer auf 127.0.0.1)
+PROXY_HOST=0.0.0.0
+PROXY_PORT=8000
+ADMIN_PORT=8001
+
+# Datenbankverbindung (Standard: SQLite für Entwicklung)
+DATABASE_URL=sqlite:///./test.db
+
+# Beispiel für PostgreSQL (Produktion):
+# DATABASE_URL=postgresql://user:password@localhost:5432/llm_quota
+
+# Ollama-Einstellungen (auch in der Admin-Oberfläche änderbar)
+OLLAMA_URL=http://localhost:11434
+DEFAULT_MODEL=llama3
+APP_TZ=Europe/Berlin