fix(smartsheet): register sync cron and fix backfill transaction visibility

Two bugs were silently breaking every Smartsheet sync since launch:

1. The /api/water-log/smartsheet-sync route was never registered in
   vercel.json's crons array. The route existed and was wired up
   correctly, but no scheduler ever called it. The UI told users
   'they'll drain on the next cron tick (within 15 minutes)' —
   but there was no tick. Queue rows accumulated indefinitely.

   Fix: register the route with schedule '*/15 * * * *'.

2. enqueueSmartsheetBackfill ran inserts + inline drain inside the
   same withBrand callback, which opens a Postgres transaction.
   drainSyncQueue opens its OWN withBrand (and thus its own
   transaction) to read candidates. Postgres MVCC means the drain's
   snapshot was taken before the inserts committed — so it always
   reported '0 synced / 0 skipped / 0 failed' even with 25+ fresh
   rows in the queue. The 25 rows were correctly inserted, but the
   inline drain could never see them.

   Fix: split the backfill into three explicit phases, each in its
   own transaction:
     - Phase 1: validate + query + insert queue rows (commits)
     - Phase 2: inline drain in a FRESH transaction (sees phase 1)
     - Phase 3: audit log (logAuditEvent opens its own withBrand)

   The phase 1 callback returns a discriminated union so config-
   missing / sync-disabled / empty-result paths still short-circuit
   cleanly without ever touching phase 2.

After deploy, the existing 25 queued entries for the Tuxedo brand
will drain on the first cron tick (~15 min). To drain immediately
without waiting:

  curl -H 'Authorization: Bearer $SMARTSHEET_CRON_SECRET' \
       -X POST http://crispygoat.com:3100/api/water-log/smartsheet-sync

vercel.json

@@ -21,6 +21,10 @@
       "path": "/api/square/process-queue",
       "schedule": "*/2 * * * *"
     },
+    {
+      "path": "/api/water-log/smartsheet-sync",
+      "schedule": "*/15 * * * *"
+    },
     {
       "path": "/api/email-automation/abandoned-cart",
       "schedule": "0 */6 * * *"