feat: add LiteLLM proxy for Anthropic Claude models

Added LiteLLM as an OpenAI-compatible proxy for Anthropic's API to
enable Claude models in Open WebUI.

**New Service: litellm**
- Image: ghcr.io/berriai/litellm:main-latest
- Internal proxy on port 4000
- Converts Anthropic API to OpenAI-compatible format
- Health check with 30s intervals
- Not exposed via Traefik (internal only)

**LiteLLM Configuration (litellm-config.yaml)**
- Claude Sonnet 4 (claude-sonnet-4-20250514)
- Claude Sonnet 4.5 (claude-sonnet-4-5-20250929)
- Claude 3.5 Sonnet (claude-3-5-sonnet-20241022)
- Claude 3 Opus (claude-3-opus-20240229)
- Claude 3 Haiku (claude-3-haiku-20240307)

**Open WebUI Configuration Updates**
- Changed OPENAI_API_BASE_URLS to point to LiteLLM proxy
- URL: http://litellm:4000/v1
- Added litellm as dependency for webui service
- Dummy API key for proxy authentication

**Why LiteLLM?**
Anthropic's API uses different endpoint structure and authentication
headers compared to OpenAI. LiteLLM acts as a translation layer,
allowing Open WebUI to use Claude models through its OpenAI-compatible
interface.

**Available Models in Open WebUI**
- claude-sonnet-4 (latest Claude Sonnet 4)
- claude-sonnet-4.5 (Claude Sonnet 4.5)
- claude-3-5-sonnet
- claude-3-opus
- claude-3-haiku

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

ai/compose.yaml

@@ -33,9 +33,9 @@ services:
       # Database configuration
       DATABASE_URL: postgresql://${AI_DB_USER}:${AI_DB_PASSWORD}@ai_postgres:5432/${AI_DB_NAME}
 
-      # OpenAI API configuration (for Claude via Anthropic API)
-      OPENAI_API_BASE_URLS: ${AI_OPENAI_API_BASE_URLS:-https://api.anthropic.com/v1}
-      OPENAI_API_KEYS: ${ANTHROPIC_API_KEY}
+      # OpenAI API configuration (pointing to LiteLLM proxy)
+      OPENAI_API_BASE_URLS: http://litellm:4000/v1
+      OPENAI_API_KEYS: sk-1234  # Dummy key for LiteLLM proxy
 
       # WebUI configuration
       WEBUI_NAME: ${AI_WEBUI_NAME:-Pivoine AI}
@@ -65,6 +65,7 @@ services:
       - ai_webui_data:/app/backend/data
     depends_on:
       - ai_postgres
+      - litellm
     networks:
       - compose_network
     labels:
@@ -86,6 +87,32 @@ services:
       # Watchtower
       - 'com.centurylinklabs.watchtower.enable=${WATCHTOWER_LABEL_ENABLE}'
 
+  # LiteLLM - Proxy to convert Anthropic API to OpenAI-compatible format
+  litellm:
+    image: ghcr.io/berriai/litellm:main-latest
+    container_name: ${AI_COMPOSE_PROJECT_NAME}_litellm
+    restart: unless-stopped
+    environment:
+      TZ: ${TIMEZONE:-Europe/Berlin}
+      ANTHROPIC_API_KEY: ${ANTHROPIC_API_KEY}
+      LITELLM_MASTER_KEY: ${AI_WEBUI_SECRET_KEY}
+    volumes:
+      - ./ai/litellm-config.yaml:/app/config.yaml:ro
+    command: ["--config", "/app/config.yaml", "--port", "4000", "--num_workers", "1"]
+    networks:
+      - compose_network
+    healthcheck:
+      test: ["CMD-SHELL", "curl -f http://localhost:4000/health || exit 1"]
+      interval: 30s
+      timeout: 10s
+      retries: 3
+      start_period: 20s
+    labels:
+      # No Traefik exposure - internal only
+      - 'traefik.enable=false'
+      # Watchtower
+      - 'com.centurylinklabs.watchtower.enable=${WATCHTOWER_LABEL_ENABLE}'
+
   # Crawl4AI - Web scraping for LLMs (internal API, no public access)
   crawl4ai:
     image: ${AI_CRAWL4AI_IMAGE:-unclecode/crawl4ai:latest}