fix(db): fail closed when WAL valve cannot checkpoint past caps (#1539) (#1751)

sync() armed the WAL valve but never called backpressure(), so daemon
catch-up could grow the WAL without bound while query-pool readers pinned
frames. Wire the writer pause into sync store + batched resolution, and
abort with WalValveAbortError after parked backfills fail past the
documented hard/file caps instead of disabling parking for 60s.

Co-authored-by: Colby McHenry <colbymchenry@users.noreply.github.com>

src/extraction/index.ts

@@ -2712,7 +2712,13 @@ export class ExtractionOrchestrator {
      * set is not exactly known (directory removals, event overflow): the full
      * scan-diff remains the ground truth those cases need (#1285).
      */
-    scopedPaths?: string[]
+    scopedPaths?: string[],
+    /**
+     * Writer-side WAL pressure valve (#1539). Called after every changed file
+     * is stored, when no extraction transaction is open, so a checkpoint can
+     * safely catch up before the next file grows the WAL further.
+     */
+    backpressure?: () => Promise<void> | null
   ): Promise<SyncResult> {
     await initGrammars(); // Initialize WASM runtime (grammars loaded lazily below)
     const startTime = Date.now();
@@ -2912,6 +2918,9 @@ export class ExtractionOrchestrator {
 
       const result = await this.indexFile(filePath);
       nodesUpdated += result.nodes.length;
+
+      const pause = backpressure?.();
+      if (pause) await pause;
     }
 
     // Names whose definition set this sync changed: a `file\0name` pair present