feat: append-only audit trail for all mutations (P0)

- Database: kanzlai.audit_log table with RLS, append-only policies
  (no UPDATE/DELETE), indexes for entity, user, and time queries
- Backend: AuditService.Log() with context-based tenant/user/IP/UA
  extraction, wired into all 7 services (case, deadline, appointment,
  document, note, party, tenant)
- API: GET /api/audit-log with entity_type, entity_id, user_id,
  from/to date, and pagination filters
- Frontend: Protokoll tab on case detail page with chronological
  audit entries, diff preview, and pagination

Required by § 50 BRAO and DSGVO Art. 5(2).

backend/internal/services/tenant_service.go

@@ -13,11 +13,12 @@ import (
 )
 
 type TenantService struct {
-	db *sqlx.DB
+	db    *sqlx.DB
+	audit *AuditService
 }
 
-func NewTenantService(db *sqlx.DB) *TenantService {
-	return &TenantService{db: db}
+func NewTenantService(db *sqlx.DB, audit *AuditService) *TenantService {
+	return &TenantService{db: db, audit: audit}
 }
 
 // Create creates a new tenant and assigns the creator as owner.
@@ -49,6 +50,7 @@ func (s *TenantService) Create(ctx context.Context, userID uuid.UUID, name, slug
 		return nil, fmt.Errorf("commit: %w", err)
 	}
 
+	s.audit.Log(ctx, "create", "tenant", &tenant.ID, nil, tenant)
 	return &tenant, nil
 }
 
@@ -171,6 +173,7 @@ func (s *TenantService) InviteByEmail(ctx context.Context, tenantID uuid.UUID, e
 		return nil, fmt.Errorf("invite user: %w", err)
 	}
 
+	s.audit.Log(ctx, "create", "membership", &tenantID, nil, ut)
 	return &ut, nil
 }
 
@@ -186,6 +189,7 @@ func (s *TenantService) UpdateSettings(ctx context.Context, tenantID uuid.UUID,
 	if err != nil {
 		return nil, fmt.Errorf("update settings: %w", err)
 	}
+	s.audit.Log(ctx, "update", "settings", &tenantID, nil, settings)
 	return &tenant, nil
 }
 
@@ -223,5 +227,6 @@ func (s *TenantService) RemoveMember(ctx context.Context, tenantID, userID uuid.
 		return fmt.Errorf("remove member: %w", err)
 	}
 
+	s.audit.Log(ctx, "delete", "membership", &tenantID, map[string]any{"user_id": userID, "role": role}, nil)
 	return nil
 }