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/schemas.py

@@ -0,0 +1,18 @@
+"""
+OpenAI-compatible models schemas.
+"""
+from src.core.models import CustomBaseModel
+
+
+class Model(CustomBaseModel):
+    """OpenAI-compatible model object."""
+    id: str
+    object: str = "model"
+    created: int
+    owned_by: str = "system"
+
+
+class ModelsResponse(CustomBaseModel):
+    """OpenAI-compatible models list response."""
+    object: str = "list"
+    data: list[Model]