feat: add file type validation to upload endpoint

- Validate filename ends with .csv (case-insensitive)
- Validate MIME type is text/* or application/csv or text/csv
- Return HTTP 400 with error message if validation fails
- Mark task 2.4 as complete

openspec/changes/code-review-fix/tasks.md

@@ -14,7 +14,7 @@
 - [x] 2.1 `[haiku]` Validate `run_id` matches `/^[a-zA-Z0-9_-]+$/` in `src/app/api/training/runs/[run_id]/route.ts` before interpolation
 - [x] 2.2 `[sonnet]` Validate `run_id` format and use `Path.resolve()` + directory containment check in `services/ml/app/main.py` (model load at line 1203, delete at line 1312)
 - [x] 2.3 `[sonnet]` Add file size check (reject >10MB) and row count limit (500,000) to `src/app/api/upload/route.ts`
-- [ ] 2.4 `[haiku]` Add file type validation (`.csv` extension, text MIME type) to `src/app/api/upload/route.ts`
+- [x] 2.4 `[haiku]` Add file type validation (`.csv` extension, text MIME type) to `src/app/api/upload/route.ts`
 - [ ] 2.5 `[haiku]` Fix CORS in `services/ml/app/main.py`: replace `allow_origins=["*"]` with `["http://localhost:3000"]` and support `CORS_ORIGINS` env var
 
 ## 3. Authentication