feat(email): HTML branded templates with inline logo, wire remaining notification types

- All webhook emails now render as branded HTML (dark header, inline base64
  Aegis logo, card layout, CTA-button links) instead of plain text.
- Wired the 7 remaining notification-preference keys that had no trigger:
  stale coverage alerts, campaign-activated assignment emails, generic
  test-state-change steps (execution started / blue evaluating / in review),
  all-team-validation broadcasts on every lead vote, webhook delivery
  failures (3rd consecutive failure), new user registration, and background
  job errors (APScheduler global error listener).
- New notify_roles_by_email() helper for role-scoped, preference-gated,
  actor-excludable broadcasts.
- Fixed apscheduler.events stubbing gaps in several test files' sys.modules
  fakes that broke full-suite collection after adding the APScheduler
  error-listener import.

backend/app/jobs/mitre_sync_job.py

@@ -16,6 +16,7 @@ from datetime import datetime, timedelta, timezone
 
 # Import BackgroundScheduler from apscheduler.schedulers.background
 from apscheduler.schedulers.background import BackgroundScheduler
+from apscheduler.events import EVENT_JOB_ERROR
 
 # Import SessionLocal from app.database
 from app.database import SessionLocal
@@ -60,6 +61,27 @@ logger = logging.getLogger(__name__)
 scheduler = BackgroundScheduler()
 
 
+def _on_job_error(event) -> None:
+    """Email admins (opted-in) whenever a scheduled background job raises."""
+    db = SessionLocal()
+    try:
+        from app.services.notification_service import notify_roles_by_email
+        notify_roles_by_email(
+            db, roles=["admin"],
+            preference_key="email_on_system_errors",
+            subject=f"Aegis Background Job Failed: {event.job_id}",
+            message=(
+                f'The scheduled job "{event.job_id}" raised an exception and did '
+                f"not complete:\n\n{event.exception}"
+            ),
+        )
+        db.commit()
+    except Exception:
+        logger.exception("Failed to dispatch system-error notification")
+    finally:
+        db.close()
+
+
 # ---------------------------------------------------------------------------
 # Job functions
 # ---------------------------------------------------------------------------
@@ -440,6 +462,7 @@ def start_scheduler() -> None:
 
     Neither job fires immediately on startup.
     """
+    scheduler.add_listener(_on_job_error, EVENT_JOB_ERROR)
     # Call scheduler.add_job()
     scheduler.add_job(
         _run_mitre_sync,