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/handlers/documents.go

@@ -167,6 +167,7 @@ func (h *DocumentHandler) Delete(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	userID, _ := auth.UserFromContext(r.Context())
+	role := auth.UserRoleFromContext(r.Context())
 
 	docID, err := uuid.Parse(r.PathValue("docId"))
 	if err != nil {
@@ -174,6 +175,26 @@ func (h *DocumentHandler) Delete(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Check permission: owner/partner can delete any, associate can delete own
+	doc, err := h.svc.GetByID(r.Context(), tenantID, docID)
+	if err != nil {
+		writeError(w, http.StatusInternalServerError, err.Error())
+		return
+	}
+	if doc == nil {
+		writeError(w, http.StatusNotFound, "document not found")
+		return
+	}
+
+	uploaderID := uuid.Nil
+	if doc.UploadedBy != nil {
+		uploaderID = *doc.UploadedBy
+	}
+	if !auth.CanDeleteDocument(role, uploaderID, userID) {
+		writeError(w, http.StatusForbidden, "insufficient permissions to delete this document")
+		return
+	}
+
 	if err := h.svc.Delete(r.Context(), tenantID, docID, userID); err != nil {
 		writeError(w, http.StatusNotFound, "document not found")
 		return