fix: cleanup stale jobs on container restart to unblock new job creation

When a Docker container is shutdown and restarted, jobs with status
'pending', 'downloading_data', or 'running' remained in the database,
preventing new jobs from starting due to concurrency control checks.

This commit adds automatic cleanup of stale jobs during FastAPI startup:

- New cleanup_stale_jobs() method in JobManager (api/job_manager.py:702-779)
- Integrated into FastAPI lifespan startup (api/main.py:164-168)
- Intelligent status determination based on completion percentage:
  - 'partial' if any model-days completed (preserves progress data)
  - 'failed' if no progress made
- Detailed error messages with original status and completion counts
- Marks incomplete job_details as 'failed' with clear error messages
- Deployment-aware: skips cleanup in DEV mode when DB is reset
- Comprehensive logging at warning level for visibility

Testing:
- 6 new unit tests covering all cleanup scenarios (451-609)
- All 30 existing job_manager tests still pass
- Tests verify pending, running, downloading_data, partial progress,
  no stale jobs, and multiple stale jobs scenarios

Resolves issue where container restarts left stale jobs blocking the
can_start_new_job() concurrency check.

api/main.py

@@ -134,25 +134,39 @@ def create_app(
     @asynccontextmanager
     async def lifespan(app: FastAPI):
         """Initialize database on startup, cleanup on shutdown if needed"""
-        from tools.deployment_config import is_dev_mode, get_db_path
+        from tools.deployment_config import is_dev_mode, get_db_path, should_preserve_dev_data
         from api.database import initialize_dev_database, initialize_database
 
         # Startup - use closure to access db_path from create_app scope
         logger.info("🚀 FastAPI application starting...")
         logger.info("📊 Initializing database...")
 
+        should_cleanup_stale_jobs = False
+
         if is_dev_mode():
             # Initialize dev database (reset unless PRESERVE_DEV_DATA=true)
             logger.info("  🔧 DEV mode detected - initializing dev database")
             dev_db_path = get_db_path(db_path)
             initialize_dev_database(dev_db_path)
             log_dev_mode_startup_warning()
+
+            # Only cleanup stale jobs if preserving dev data (otherwise DB is fresh)
+            if should_preserve_dev_data():
+                should_cleanup_stale_jobs = True
         else:
             # Ensure production database schema exists
             logger.info("  🏭 PROD mode - ensuring database schema exists")
             initialize_database(db_path)
+            should_cleanup_stale_jobs = True
 
         logger.info("✅ Database initialized")
+
+        # Clean up stale jobs from previous container session
+        if should_cleanup_stale_jobs:
+            logger.info("🧹 Checking for stale jobs from previous session...")
+            job_manager = JobManager(get_db_path(db_path) if is_dev_mode() else db_path)
+            job_manager.cleanup_stale_jobs()
+
         logger.info("🌐 API server ready to accept requests")
 
         yield