perf: implement IndexedDB connection pooling in dmDb.js

Refactor `js/storage/dmDb.js` to use a singleton connection pattern for IndexedDB.
Previously, the database was opened and closed for every single operation,
leading to significant overhead.

Changes:
- Introduced `dbInstance` and `dbPromise` to cache the database connection.
- Refactored `openDmDb` to reuse the existing connection or pending promise.
- Added `onversionchange` handler to safely close the connection if an upgrade is requested.
- Removed all manual `db.close()` calls from exported functions.
- Added a benchmark script in `tests/benchmark-dm-db.mjs` to verify the improvement.

Co-authored-by: PR0M3TH3AN <[email protected]>

tests/benchmark-dm-db.mjs

@@ -0,0 +1,44 @@
+import { writeMessages } from "../js/storage/dmDb.js";
+import { indexedDB, IDBKeyRange } from "fake-indexeddb";
+
+if (!globalThis.indexedDB) {
+  globalThis.indexedDB = indexedDB;
+  globalThis.IDBKeyRange = IDBKeyRange;
+}
+
+const ITERATIONS = 500;
+
+async function benchmark() {
+  console.log(`Starting benchmark with ${ITERATIONS} iterations...`);
+
+  // Warm up
+  await writeMessages({
+    id: `warmup`,
+    conversationId: "bench-conv",
+    senderPubkey: "sender",
+    receiverPubkey: "receiver",
+    createdAt: Date.now(),
+    kind: 4,
+    content: "Warmup",
+  });
+
+  const start = performance.now();
+
+  for (let i = 0; i < ITERATIONS; i++) {
+    await writeMessages({
+      id: `msg-${i}`,
+      conversationId: "bench-conv",
+      senderPubkey: "sender",
+      receiverPubkey: "receiver",
+      createdAt: Date.now(),
+      kind: 4,
+      content: "Bench",
+    });
+  }
+
+  const end = performance.now();
+  console.log(`Total time for ${ITERATIONS} writes: ${(end - start).toFixed(2)}ms`);
+  console.log(`Average time per write: ${((end - start) / ITERATIONS).toFixed(2)}ms`);
+}
+
+benchmark().catch(console.error);