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/auth/tenant_resolver_test.go

@@ -11,6 +11,7 @@ import (
 
 type mockTenantLookup struct {
 	tenantID *uuid.UUID
+	role     string
 	err      error
 }
 
@@ -18,9 +19,16 @@ func (m *mockTenantLookup) FirstTenantForUser(ctx context.Context, userID uuid.U
 	return m.tenantID, m.err
 }
 
+func (m *mockTenantLookup) GetUserRole(ctx context.Context, userID, tenantID uuid.UUID) (string, error) {
+	if m.role != "" {
+		return m.role, m.err
+	}
+	return "associate", m.err
+}
+
 func TestTenantResolver_FromHeader(t *testing.T) {
 	tenantID := uuid.New()
-	tr := NewTenantResolver(&mockTenantLookup{})
+	tr := NewTenantResolver(&mockTenantLookup{role: "partner"})
 
 	var gotTenantID uuid.UUID
 	next := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {