feat(auth): remove username field, migrate identity to email

- AppUser entity: replace username with email (NOT NULL, UNIQUE,
  colon-pattern validated)
- AppUserRepository: remove findByUsername, rename search JPQL to
  searchByEmailOrName (searches email + firstName + lastName)
- CreateUserRequest: remove username, require email with colon guard
- UserService: rename findByUsername→findByEmail, createUserOrUpdate
  upserts by email, blank-email guard throws instead of setting null
- UserController + all other controllers: findByEmail(auth.getName())
- DataInitializer: email-based config and lookup, E2E users have email
- V44 migration: pre-check + email NOT NULL + drop username column
- All tests updated: .username() builders removed, mocks updated,
  NotificationRepositoryTest fixtures include email fields

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/resources/db/migration/V44__email_only_auth.sql

@@ -0,0 +1,11 @@
+-- Abort if any user has no email address set.
+-- All users must have an email before this migration can run.
+DO $$
+BEGIN
+  IF EXISTS (SELECT 1 FROM users WHERE email IS NULL) THEN
+    RAISE EXCEPTION 'Migration aborted: some users have no email address. Set emails for all users before running this migration.';
+  END IF;
+END $$;
+
+ALTER TABLE users ALTER COLUMN email SET NOT NULL;
+ALTER TABLE users DROP COLUMN username;