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/services/test_workflow_service.py

@@ -39,6 +39,7 @@ from app.services.notification_service import (
     notify_test_state_change,
     create_notification,
     notify_role_with_email,
+    notify_roles_by_email,
 )
 
 # Assign logger = logging.getLogger(__name__)
@@ -1048,6 +1049,13 @@ def validate_as_red_lead(
         },
     )
 
+    notify_roles_by_email(
+        db, roles=["red_lead", "blue_lead"],
+        preference_key="email_on_all_team_validations",
+        subject=f"Red Lead Vote: {test.name}",
+        message=f'{user.full_name or user.username} cast a "{validation_status}" vote as Red Lead on test "{test.name}".',
+        exclude_user_id=user.id,
+    )
     _dispatch_dual_validation_effects(db, test, entity, actor=user)
     return test
 
@@ -1113,6 +1121,13 @@ def validate_as_blue_lead(
         },
     )
 
+    notify_roles_by_email(
+        db, roles=["red_lead", "blue_lead"],
+        preference_key="email_on_all_team_validations",
+        subject=f"Blue Lead Vote: {test.name}",
+        message=f'{user.full_name or user.username} cast a "{validation_status}" vote as Blue Lead on test "{test.name}".',
+        exclude_user_id=user.id,
+    )
     _dispatch_dual_validation_effects(db, test, entity, actor=user)
     return test