perf(db): batch node lookups, fix insertNode cache, run maintenance after writes (#108)

Batch getNodesByIds to collapse N+1 reads in graph traversal, invalidate the
insertNode LRU cache so INSERT OR REPLACE doesn't serve a stale row, and run
incremental PRAGMA optimize + passive WAL checkpoint after bulk writes.

Closes #108

src/db/index.ts

@@ -186,6 +186,36 @@ export class DatabaseConnection {
     this.db.exec('ANALYZE');
   }
 
+  /**
+   * Lightweight, non-blocking maintenance to run after bulk writes
+   * (indexAll, sync). Two operations:
+   *
+   *   - `PRAGMA optimize` — incremental ANALYZE; SQLite only re-analyzes
+   *     tables whose row counts changed materially since the last
+   *     ANALYZE. Without it, the query planner has no statistics on the
+   *     freshly-bulk-loaded tables and can pick suboptimal indexes.
+   *
+   *   - `PRAGMA wal_checkpoint(PASSIVE)` — fold pending WAL pages back
+   *     into the main database file so the WAL file doesn't grow
+   *     unboundedly between automatic checkpoints (auto-fires at 1000
+   *     pages by default; large indexAll runs blow past that).
+   *
+   * Both operations are silently swallowed on failure — they're a
+   * best-effort optimization, never load-bearing for correctness.
+   */
+  runMaintenance(): void {
+    try {
+      this.db.exec('PRAGMA optimize');
+    } catch {
+      // ignore
+    }
+    try {
+      this.db.exec('PRAGMA wal_checkpoint(PASSIVE)');
+    } catch {
+      // ignore (e.g., not in WAL mode)
+    }
+  }
+
   /**
    * Close the database connection
    */