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/tests/test_password_setup_flow.py

@@ -74,7 +74,7 @@ def test_send_password_email_posts_to_webhook_and_issues_token(api, db, auth_hea
     payload = call_kwargs.kwargs["json"]
     assert payload["to"] == "setpw@test.com"
     assert payload["subject"] == "Set Your Password"
-    assert "HI Set Password User" in payload["body"]
+    assert "Hi Set Password User" in payload["body"]
     assert "token=" in payload["body"]
     assert "Purple Team Engineering" in payload["body"]