perf(simplefin): batch sync writes in a transaction + align dedup key

- Wrap each source's account/transaction upserts + the status update in one
  better-sqlite3 db.transaction() — a single commit instead of one implicit
  commit per row (far fewer fsyncs on large syncs) and atomic (a mid-sync
  failure can't leave a half-written account). The write block is synchronous;
  the async SimpleFIN fetch already completed before it.
- Key the dedup lookup on (user_id, provider_transaction_id) to match the UNIQUE
  index and the reconnect-stable provider id. A data_source_id-keyed lookup
  missed the prior row after a disconnect/reconnect, so pending→posted and
  amount-refresh were skipped; now they survive a reconnect.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

services/bankSyncService.js

@@ -64,10 +64,14 @@ function upsertAccount(db, accountRow) {
 // a row the user has matched or ignored is never touched.
 // Returns: 'inserted' | 'posted' (pending→settled) | 'updated' (pending refreshed) | 'skipped'.
 function upsertTransaction(db, txRow) {
+  // Key on (user_id, provider_transaction_id) to match the UNIQUE dedup index —
+  // provider_transaction_id is deliberately stable across disconnect/reconnect, so
+  // this also lets the pending→posted / amount-refresh paths survive a reconnect
+  // (a data_source_id-keyed lookup would miss the prior row and skip the update).
   const existing = db.prepare(`
     SELECT id, pending, match_status FROM transactions
-    WHERE data_source_id = ? AND provider_transaction_id = ?
+    WHERE user_id = ? AND provider_transaction_id = ?
-  `).get(txRow.data_source_id, txRow.provider_transaction_id);
+  `).get(txRow.user_id, txRow.provider_transaction_id);
 
   if (!existing) {
     try {
@@ -145,6 +149,22 @@ async function runSync(db, userId, dataSource, { days, debug = false } = {}) {
       AND provider_transaction_id NOT IN (SELECT value FROM json_each(?))
   `);
 
+  // Store any errlist warnings alongside a successful sync so users can see them
+  const partialError = raw._errlistSummary
+    ? sanitizeErrorMessage(`Partial sync — some connections failed: ${raw._errlistSummary}`)
+    : null;
+
+  const updateSource = db.prepare(`
+    UPDATE data_sources
+    SET last_sync_at = datetime('now'), last_error = ?, status = 'active', updated_at = datetime('now')
+    WHERE id = ? AND user_id = ?
+  `);
+
+  // Batch every account + transaction write and the source-status update into ONE
+  // commit (better-sqlite3 db.transaction): far fewer fsyncs on large syncs, and
+  // atomic — a mid-sync failure can't leave a half-written account. The whole
+  // block is synchronous (the async fetch already happened above).
+  const ingest = db.transaction(() => {
     for (const rawAccount of accounts) {
       const accountRow   = normalizeAccount(rawAccount, dataSource.id, userId);
       const localAccount = upsertAccount(db, accountRow);
@@ -183,16 +203,9 @@ async function runSync(db, userId, dataSource, { days, debug = false } = {}) {
       }
     }
 
-  // Store any errlist warnings alongside a successful sync so users can see them
+    updateSource.run(partialError, dataSource.id, userId);
-  const partialError = raw._errlistSummary
+  });
-    ? sanitizeErrorMessage(`Partial sync — some connections failed: ${raw._errlistSummary}`)
+  ingest();
-    : null;
-
-  db.prepare(`
-    UPDATE data_sources
-    SET last_sync_at = datetime('now'), last_error = ?, status = 'active', updated_at = datetime('now')
-    WHERE id = ? AND user_id = ?
-  `).run(partialError, dataSource.id, userId);
 
   if (debug) console.log(`[bankSync:debug] Source #${dataSource.id}: applying merchant rules + auto-match`);