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/litellm-config.yaml

@@ -0,0 +1,29 @@
+model_list:
+  - model_name: claude-sonnet-4
+    litellm_params:
+      model: anthropic/claude-sonnet-4-20250514
+      api_key: ${ANTHROPIC_API_KEY}
+
+  - model_name: claude-sonnet-4.5
+    litellm_params:
+      model: anthropic/claude-sonnet-4-5-20250929
+      api_key: ${ANTHROPIC_API_KEY}
+
+  - model_name: claude-3-5-sonnet
+    litellm_params:
+      model: anthropic/claude-3-5-sonnet-20241022
+      api_key: ${ANTHROPIC_API_KEY}
+
+  - model_name: claude-3-opus
+    litellm_params:
+      model: anthropic/claude-3-opus-20240229
+      api_key: ${ANTHROPIC_API_KEY}
+
+  - model_name: claude-3-haiku
+    litellm_params:
+      model: anthropic/claude-3-haiku-20240307
+      api_key: ${ANTHROPIC_API_KEY}
+
+litellm_settings:
+  drop_params: true
+  set_verbose: false