fix(m4): typed MitreSyncResult interface — drop the as cast

Mirrors the backend Pydantic `SyncResultOut` in TS so the mutation result is
properly typed end-to-end. `(res as { duration_ms: number })` cast removed
from MitrePage.tsx; `apiPost<MitreSyncResult>` carries the contract.

Also annotated the unused query-key factories in mitre.ts so the next reader
knows they're parked for M5 template-form consumption (not dead).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/lib/mitre.ts

@@ -51,6 +51,10 @@ export interface MitreTag {
   name: string;
 }
 
+// Query keys. `status` + `matrix` drive the M4 picker; the per-list factories
+// (`tactics`/`techniques`/`subtechniques`) are unused today but the M5
+// template forms will consume them for the standalone REST endpoints when
+// users edit a single test's tags inline.
 export const mitreKeys = {
   status: ['mitre', 'status'] as const,
   matrix: ['mitre', 'matrix'] as const,
@@ -85,3 +89,17 @@ export interface MatrixTactic {
 export interface MitreMatrix {
   tactics: MatrixTactic[];
 }
+
+/** Mirror of backend `SyncResultOut` (`api/mitre.py`). */
+export interface MitreSyncResult {
+  tactics_upserted: number;
+  techniques_upserted: number;
+  subtechniques_upserted: number;
+  subtechniques_skipped_orphan: number;
+  technique_tactic_links: number;
+  version: string | null;
+  source: string;
+  started_at: string;
+  finished_at: string;
+  duration_ms: number;
+}