migration 083: use CREATE INDEX CONCURRENTLY to avoid locking fmcsa_carriers

The original CREATE INDEX (non-concurrent) on a 2M-row table held a SHARE lock
for ~33 minutes, blocking all 25+ DOT checker queries and causing 'Failed to
fetch' for real users. CONCURRENTLY builds the index without a table lock.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

api/migrations/083_fmcsa_campaign_tracking.sql

@@ -5,6 +5,6 @@ ALTER TABLE fmcsa_carriers
   ADD COLUMN IF NOT EXISTS listmonk_sent_at TIMESTAMPTZ,
   ADD COLUMN IF NOT EXISTS listmonk_campaign_type TEXT;
 
-CREATE INDEX IF NOT EXISTS idx_fmcsa_carriers_listmonk_sent
+CREATE INDEX CONCURRENTLY IF NOT EXISTS idx_fmcsa_carriers_listmonk_sent
   ON fmcsa_carriers (listmonk_sent_at)
   WHERE listmonk_sent_at IS NULL;