feat(US7+US8): finetune management and health check test

- app/models/finetune_models.py: FinetuneStartRequest, FinetuneStartResponse, FinetuneStatusResponse
- app/services/finetune_service.py: submit_finetune + get_finetune_status via run_in_executor; status map running→RUNNING, succeeded→SUCCESS, failed→FAILED, unknown→RUNNING; LLMCallError on SDK failure
- app/routers/finetune.py: POST /finetune/start + GET /finetune/status/{job_id} with get_llm_client dependency
- tests/test_finetune_service.py: 12 unit tests (TDD, written before implementation)
- tests/test_finetune_router.py: 6 integration tests
- tests/test_health.py: GET /health → 200 {"status":"ok"}

Full suite: 72/72 passing (was 53)

app/models/finetune_models.py

@@ -0,0 +1,18 @@
+from pydantic import BaseModel
+
+
+class FinetuneStartRequest(BaseModel):
+    jsonl_url: str
+    base_model: str
+    hyperparams: dict | None = None
+
+
+class FinetuneStartResponse(BaseModel):
+    job_id: str
+
+
+class FinetuneStatusResponse(BaseModel):
+    job_id: str
+    status: str
+    progress: int | None = None
+    error_message: str | None = None