Scope training run queries in FastAPI to filter by user ID (Task 14.3)

- Add user_id column to TrainingRun model in db.py
- Store user_id on TrainingRun insert in /training/start
- Filter GET /training/runs by user_id (returns empty list if no user context)
- Enforce user ownership on GET /training/runs/{run_id} (404 on mismatch)
- Enforce user ownership on DELETE /training/runs/{run_id} (404 on mismatch)
- Add migration 002 to add user_id column and index to training_runs table

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

services/ml/app/db.py

@@ -46,6 +46,7 @@ class TrainingRun(Base):
 
     id = Column(Integer, primary_key=True, index=True)
     run_id = Column(String(255), unique=True, nullable=False, index=True)
+    user_id = Column(String(255), nullable=True, index=True)
     model_type = Column(String(100), nullable=False)
     experiment_name = Column(String(255), nullable=False, index=True)
     pipeline_config_hash = Column(String(64), nullable=False)