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/tenant_service.go

@@ -189,6 +189,40 @@ func (s *TenantService) UpdateSettings(ctx context.Context, tenantID uuid.UUID,
 	return &tenant, nil
 }
 
+// UpdateMemberRole changes a member's role in a tenant.
+func (s *TenantService) UpdateMemberRole(ctx context.Context, tenantID, userID uuid.UUID, newRole string) error {
+	// Get current role
+	currentRole, err := s.GetUserRole(ctx, userID, tenantID)
+	if err != nil {
+		return fmt.Errorf("get current role: %w", err)
+	}
+	if currentRole == "" {
+		return fmt.Errorf("user is not a member of this tenant")
+	}
+
+	// If demoting the last owner, block it
+	if currentRole == "owner" && newRole != "owner" {
+		var ownerCount int
+		err := s.db.GetContext(ctx, &ownerCount,
+			`SELECT COUNT(*) FROM user_tenants WHERE tenant_id = $1 AND role = 'owner'`,
+			tenantID)
+		if err != nil {
+			return fmt.Errorf("count owners: %w", err)
+		}
+		if ownerCount <= 1 {
+			return fmt.Errorf("cannot demote the last owner")
+		}
+	}
+
+	_, err = s.db.ExecContext(ctx,
+		`UPDATE user_tenants SET role = $1 WHERE user_id = $2 AND tenant_id = $3`,
+		newRole, userID, tenantID)
+	if err != nil {
+		return fmt.Errorf("update role: %w", err)
+	}
+	return nil
+}
+
 // RemoveMember removes a user from a tenant. Cannot remove the last owner.
 func (s *TenantService) RemoveMember(ctx context.Context, tenantID, userID uuid.UUID) error {
 	// Check if the user being removed is an owner