feat: role-based permissions (owner/partner/associate/paralegal/secretary)

Backend:
- auth/permissions.go: full permission matrix with RequirePermission/RequireRole
  middleware, CanEditCase, CanDeleteDocument helpers
- auth/context.go: add user role to request context
- auth/middleware.go: resolve role alongside tenant in auth flow
- auth/tenant_resolver.go: verify membership + resolve role for X-Tenant-ID
- handlers/case_assignments.go: CRUD for case-level user assignments
- handlers/tenant_handler.go: UpdateMemberRole, GetMe (/api/me) endpoints
- handlers/documents.go: permission-based delete (own vs all)
- router/router.go: permission-wrapped routes for all endpoints
- services/case_assignment_service.go: assign/unassign with tenant validation
- services/tenant_service.go: UpdateMemberRole with owner protection
- models/case_assignment.go: CaseAssignment model

Database:
- user_tenants.role: CHECK constraint (owner/partner/associate/paralegal/secretary)
- case_assignments table: case_id, user_id, role (lead/team/viewer)
- Migrated existing admin->partner, member->associate

Frontend:
- usePermissions hook: fetches /api/me, provides can() helper
- TeamSettings: 5-role dropdown, role change, permission-gated invite
- CaseAssignments: new component for case-level team management
- Sidebar: conditionally hides AI/Settings based on permissions
- Cases page: hides "Neue Akte" button for non-authorized roles
- Case detail: new "Mitarbeiter" tab for assignment management

backend/internal/services/case_assignment_service.go

@@ -0,0 +1,92 @@
+package services
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/google/uuid"
+	"github.com/jmoiron/sqlx"
+
+	"mgit.msbls.de/m/KanzlAI-mGMT/internal/models"
+)
+
+type CaseAssignmentService struct {
+	db *sqlx.DB
+}
+
+func NewCaseAssignmentService(db *sqlx.DB) *CaseAssignmentService {
+	return &CaseAssignmentService{db: db}
+}
+
+// ListByCase returns all assignments for a case.
+func (s *CaseAssignmentService) ListByCase(ctx context.Context, tenantID, caseID uuid.UUID) ([]models.CaseAssignment, error) {
+	var assignments []models.CaseAssignment
+	err := s.db.SelectContext(ctx, &assignments,
+		`SELECT ca.id, ca.case_id, ca.user_id, ca.role, ca.assigned_at
+		 FROM case_assignments ca
+		 JOIN cases c ON c.id = ca.case_id
+		 WHERE ca.case_id = $1 AND c.tenant_id = $2
+		 ORDER BY ca.assigned_at`,
+		caseID, tenantID)
+	if err != nil {
+		return nil, fmt.Errorf("list case assignments: %w", err)
+	}
+	return assignments, nil
+}
+
+// Assign adds a user to a case with the given role.
+func (s *CaseAssignmentService) Assign(ctx context.Context, tenantID, caseID, userID uuid.UUID, role string) (*models.CaseAssignment, error) {
+	// Verify user is a member of this tenant
+	var memberExists bool
+	err := s.db.GetContext(ctx, &memberExists,
+		`SELECT EXISTS(SELECT 1 FROM user_tenants WHERE user_id = $1 AND tenant_id = $2)`,
+		userID, tenantID)
+	if err != nil {
+		return nil, fmt.Errorf("check membership: %w", err)
+	}
+	if !memberExists {
+		return nil, fmt.Errorf("user is not a member of this tenant")
+	}
+
+	// Verify case belongs to tenant
+	var caseExists bool
+	err = s.db.GetContext(ctx, &caseExists,
+		`SELECT EXISTS(SELECT 1 FROM cases WHERE id = $1 AND tenant_id = $2)`,
+		caseID, tenantID)
+	if err != nil {
+		return nil, fmt.Errorf("check case: %w", err)
+	}
+	if !caseExists {
+		return nil, fmt.Errorf("case not found")
+	}
+
+	var assignment models.CaseAssignment
+	err = s.db.QueryRowxContext(ctx,
+		`INSERT INTO case_assignments (case_id, user_id, role)
+		 VALUES ($1, $2, $3)
+		 ON CONFLICT (case_id, user_id) DO UPDATE SET role = EXCLUDED.role
+		 RETURNING id, case_id, user_id, role, assigned_at`,
+		caseID, userID, role,
+	).StructScan(&assignment)
+	if err != nil {
+		return nil, fmt.Errorf("assign user to case: %w", err)
+	}
+	return &assignment, nil
+}
+
+// Unassign removes a user from a case.
+func (s *CaseAssignmentService) Unassign(ctx context.Context, tenantID, caseID, userID uuid.UUID) error {
+	result, err := s.db.ExecContext(ctx,
+		`DELETE FROM case_assignments ca
+		 USING cases c
+		 WHERE ca.case_id = c.id AND ca.case_id = $1 AND ca.user_id = $2 AND c.tenant_id = $3`,
+		caseID, userID, tenantID)
+	if err != nil {
+		return fmt.Errorf("unassign: %w", err)
+	}
+	rows, _ := result.RowsAffected()
+	if rows == 0 {
+		return fmt.Errorf("assignment not found")
+	}
+	return nil
+}