Auto-commit: v2.14 — Stuck job robustness (heartbeat, retry, stale watcher, granular logging)

- web/db.py: Add last_heartbeat column to Run model
- core/utils.py: Add set_heartbeat_callback() and send_heartbeat()
- web/tasks.py: Add _robust_update_run_status() with 5-retry exponential backoff;
  add db_heartbeat_callback(); remove all bare except:pass on DB status updates;
  set start_time + last_heartbeat when marking run as 'running'
- web/app.py: Add last_heartbeat column migration; add _stale_job_watcher()
  background thread (checks every 5 min, 15-min heartbeat threshold, 2-hr start_time threshold)
- cli/engine.py: Add phase-level logging banners and try/except wrappers in
  process_book(); add utils.send_heartbeat() after each chapter save;
  add start/finish logging in run_generation()

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

web/db.py

@@ -33,6 +33,7 @@ class Run(db.Model):
     log_file = db.Column(db.String(300), nullable=True)
     cost = db.Column(db.Float, default=0.0)
     progress = db.Column(db.Integer, default=0)
+    last_heartbeat = db.Column(db.DateTime, nullable=True)
 
     logs = db.relationship('LogEntry', backref='run', lazy=True, cascade="all, delete-orphan")