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/campaigns.py

@@ -124,7 +124,7 @@ from app.services.campaign_crud_service import (
 from app.services.audit_service import log_action
 
 # Import notify_role from app.services.notification_service
-from app.services.notification_service import notify_role
+from app.services.notification_service import notify_role, notify_roles_by_email
 from app.services.webhook_service import dispatch_webhook
 
 # Assign logger = logging.getLogger(__name__)
@@ -543,6 +543,12 @@ def approve_campaign_endpoint(
             entity_id=campaign.id,
             details={"start_date": payload.start_date},
         )
+        notify_roles_by_email(
+            db, roles=["red_tech"],
+            preference_key="email_on_assigned_to_campaign",
+            subject=f"Campaign Activated: {campaign.name}",
+            message=f'Campaign "{campaign.name}" has been approved and activated. You may have tests assigned.',
+        )
         uow.commit()
     db.refresh(campaign)
 
@@ -868,6 +874,12 @@ def activate_campaign(
             # Keyword argument: entity_id
             entity_id=campaign.id,
         )
+        notify_roles_by_email(
+            db, roles=["red_tech"],
+            preference_key="email_on_assigned_to_campaign",
+            subject=f"Campaign Activated: {campaign.name}",
+            message=f'Campaign "{campaign.name}" has been activated. You may have tests assigned.',
+        )
         # Call log_action()
         log_action(
             db,