Add Anthropic Messages API compatibility layer (/v1/messages)

- POST /v1/messages endpoint with full quota enforcement and auth
- Accepts x-api-key and anthropic-auth-token headers (for Claude Code)
- Transforms Anthropic request/response format ↔ Ollama /api/chat
- Streaming support via Anthropic SSE format
- Tool use support (request and response transformation)
- ANTHROPIC_DEFAULT_MODEL env var for model selection without admin UI
- BACKEND_API_KEY env var for forwarding auth to upstream proxies
- Fix SQLite path always resolved relative to database.py location
- start.sh and start_claude.sh load .env relative to script location

.env.example

@@ -16,3 +16,8 @@ DATABASE_URL=sqlite:///./test.db
 OLLAMA_URL=http://localhost:11434
 DEFAULT_MODEL=llama3
 APP_TZ=Europe/Berlin
+
+# Standard-Modell für den Anthropic-kompatiblen Endpunkt (/v1/messages)
+# Wird verwendet, wenn der Client kein Modell angibt oder ein Anthropic-Modellname
+# (z.B. claude-opus-4-7) auf kein lokales Modell passt.
+ANTHROPIC_DEFAULT_MODEL=llama3