fix(migrations): re-issue the 12xx label backfill inside an explicit transaction block (#2422)

* fix(migrations): re-issue the 12xx label backfill inside an explicit transaction block

20260908113353 (PR #2419) used a bare LOCK TABLE. The CI replay
(psql -f per file) and the Supabase branch runner execute migration
statements in autocommit, so Postgres refused it ("LOCK TABLE can only
be used in transaction blocks"): pg-real, pg-upgrade and tool-pg went
red on main and prod's migration queue stopped at that version, which
also blocks 20260908130127 (PR #2420). Prod never recorded 113353, so
the file is replaced rather than edited: same statements wrapped in
BEGIN/COMMIT (precedent 20260513140000), new version 20260908120449.
Applied and pg-tested on staging.

Refs #2413

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NXSuVejFCvRDyNXF1otEPd

* docs(decisions): migrations wrap transaction-only statements in BEGIN/COMMIT

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NXSuVejFCvRDyNXF1otEPd

---------

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

tests/pg/bas2026-retired-12xx-labels.pg.test.ts

@@ -5,7 +5,7 @@ import { insertPostedJournalEntry, seedCompany } from '@/tests/pg/fixtures'
 import { getPool } from '@/tests/pg/setup'
 
 /**
- * pg-real coverage for 20260908113353_backfill_bas2026_retired_12xx_labels.sql
+ * pg-real coverage for 20260908120449_backfill_bas2026_retired_12xx_labels.sql
  * (#2413).
  *
  * The backfill renames 1249/1259/1269 only where the chart carries the
@@ -22,7 +22,7 @@ import { getPool } from '@/tests/pg/setup'
  */
 
 const BACKFILL_SQL = readFileSync(
-  join(process.cwd(), 'supabase/migrations/20260908113353_backfill_bas2026_retired_12xx_labels.sql'),
+  join(process.cwd(), 'supabase/migrations/20260908120449_backfill_bas2026_retired_12xx_labels.sql'),
   'utf8',
 )