refactor(db): lazy-init connection and add test seam

The Database handle is now constructed on first use and held in
a module-level slot. New _setDbForTest and _resetDbForTest
exports let tests swap in a :memory: handle without rewriting
callers. closeDb() replaces the previous db.close() pattern used
by short-lived scripts. Adds listDistinctNoteDids() to keep the
backfill script from poking the raw connection.

scripts/backfill-opensearch.ts

@@ -9,7 +9,7 @@
 // Requires OPENSEARCH_URL to be set; falls back to a no-op otherwise.
 // SQLite path follows the same SQLITE_PATH convention as the server/jetstream.
 
-import { db } from "../src/data/db.ts";
+import { closeDb, listDistinctNoteDids } from "../src/data/db.ts";
 import { resolvePds } from "../src/auth/verify.ts";
 import { ensureIndex, indexNote } from "../src/search/opensearch.ts";
 import { log } from "../src/log.ts";
@@ -100,14 +100,12 @@ const main = async () => {
     Deno.exit(1);
   }
 
-  const rows = db.prepare("SELECT DISTINCT did FROM note ORDER BY did").all<
-    { did: string }
-  >();
-  log(`[backfill] ${rows.length} DID(s) to process`);
+  const dids = listDistinctNoteDids();
+  log(`[backfill] ${dids.length} DID(s) to process`);
 
   let totalIndexed = 0;
   let totalFailed = 0;
-  for (const { did } of rows) {
+  for (const did of dids) {
     try {
       const { indexed, failed } = await backfillDid(did);
       log(`[backfill] ${did}: indexed=${indexed} failed=${failed}`);
@@ -119,7 +117,7 @@ const main = async () => {
     }
   }
   log(`[backfill] done. indexed=${totalIndexed} failed=${totalFailed}`);
-  db.close();
+  closeDb();
 };
 
 await main();