feat(backend): derive LEITUNGSTEAM from the Authentik groups claim

On every Authentik login the token's `groups` claim is compared against
AUTHENTIK_LEITUNGSTEAM_GROUP (default "Leitungsteam") and mirrored to the
new User.isLeitungsteam column. LT is global, not KC-scoped, so it lives on
the User rather than as a per-KC Membership row: toAuthenticatedUser()
synthesises a virtual global LEITUNGSTEAM membership from the flag, so
RolesGuard / visibility / TeamerService keep working unchanged.

- provision helper gains an isLeitungsteam arg and reconciles the flag both
  ways (grant on join, drop when the group is gone), capturing a User
  UPDATE to the sync log.
- verifyAuthentikClaims() now also returns isLeitungsteam; strategy, WS
  path and onboarding all funnel through the shared helper + mapper.
- new env var AUTHENTIK_LEITUNGSTEAM_GROUP.

Tests: provision-user.spec.ts extended (flag up/down, virtual membership);
npm test green at 55. Docs updated; ops note added that the Authentik
provider must emit the groups claim.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/prisma/schema.prisma

@@ -60,14 +60,19 @@ enum MembershipStatus {
 /// Teamer are local accounts created by a Verantwortliche/r (`passwordHash`
 /// set, `authentikSub` null, `kcId` set) and, like guests, scoped to one KC.
 model User {
-  id           String   @id @default(cuid())
-  authentikSub String?  @unique
-  email        String   @unique
-  firstName    String
-  lastName     String
-  passwordHash String?
-  kcId         String?
-  createdAt    DateTime @default(now())
+  id             String   @id @default(cuid())
+  authentikSub   String?  @unique
+  email          String   @unique
+  firstName      String
+  lastName       String
+  passwordHash   String?
+  kcId           String?
+  /// Mirrored from the caller's Authentik group membership on every login.
+  /// LEITUNGSTEAM is global (not KC-scoped), so it lives here rather than as
+  /// a per-KC Membership row; the auth layer synthesises a virtual global
+  /// LEITUNGSTEAM membership from this flag.
+  isLeitungsteam Boolean  @default(false)
+  createdAt      DateTime @default(now())
 
   kc                 Kc?               @relation(fields: [kcId], references: [id], onDelete: Cascade)
   memberships        Membership[]