Implement models listing domain

Add OpenAI-compatible models listing endpoint.
Currently returns mock model (mistral-nemo:latest).

Models Router (src/models/router.py):
- GET /v1/models endpoint
- OpenAI-compatible response format
- Lists available models

Models Schemas (src/models/schemas.py):
- Model object with id, created, owned_by
- ModelsListResponse with data array
- Full OpenAI API compatibility

Models Service (src/models/service.py):
- list_models() function
- Mock model listing (ready for Ollama integration)
- Returns mistral-nemo:latest as default

Following Best Practices:
- Business logic in service layer
- Router only handles HTTP concerns
- Type hints throughout
- Async/await pattern

Model: mistral-nemo:latest
Status: Mock implementation (ready for Ollama integration)

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

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

src/models/service.py

@@ -0,0 +1,34 @@
+"""
+Models service.
+Currently returns mock model list.
+TODO: Fetch from Ollama in future.
+"""
+import time
+
+from src.models.schemas import Model, ModelsResponse
+
+
+# Mock models (will be replaced with Ollama models later)
+MOCK_MODELS = [
+    {"id": "mistral-nemo:latest", "created": int(time.time())},
+]
+
+
+async def list_models() -> ModelsResponse:
+    """
+    List available models (mock implementation).
+    
+    Returns:
+        Mock list of models
+    """
+    models = [
+        Model(
+            id=model["id"],
+            object="model",
+            created=model["created"],
+            owned_by="system",
+        )
+        for model in MOCK_MODELS
+    ]
+    
+    return ModelsResponse(object="list", data=models)