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/routers/finetune.py

@@ -1,3 +1,28 @@
-from fastapi import APIRouter
+from fastapi import APIRouter, Depends
+
+from app.clients.llm.base import LLMClient
+from app.core.dependencies import get_llm_client
+from app.models.finetune_models import (
+    FinetuneStartRequest,
+    FinetuneStartResponse,
+    FinetuneStatusResponse,
+)
+from app.services import finetune_service
 
 router = APIRouter(tags=["Finetune"])
+
+
+@router.post("/finetune/start", response_model=FinetuneStartResponse)
+async def start_finetune(
+    req: FinetuneStartRequest,
+    llm: LLMClient = Depends(get_llm_client),
+) -> FinetuneStartResponse:
+    return await finetune_service.submit_finetune(req, llm)
+
+
+@router.get("/finetune/status/{job_id}", response_model=FinetuneStatusResponse)
+async def get_status(
+    job_id: str,
+    llm: LLMClient = Depends(get_llm_client),
+) -> FinetuneStatusResponse:
+    return await finetune_service.get_finetune_status(job_id, llm)