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/routers/users.py

@@ -185,6 +185,18 @@ def create_user_route(
     # Reload ORM object attributes from the database
     db.refresh(user)
 
+    from app.services.notification_service import notify_roles_by_email
+    notify_roles_by_email(
+        db, roles=["admin"],
+        preference_key="email_on_new_users",
+        subject=f"New User Created: {user.full_name or user.email}",
+        message=(
+            f'A new user account was created: {user.full_name or user.email} '
+            f"({user.email}), role: {user.role}."
+        ),
+        exclude_user_id=current_user.id,
+    )
+
     # Return user
     return user