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>

supabase/migrations/20260908120449_backfill_bas2026_retired_12xx_labels.sql

@@ -26,11 +26,16 @@
 -- corrects the label the picker handed out before anything was booked on it.
 -- No row is deleted; bookings key on account_number, never on the label.
 --
--- The migration runs in one transaction (Supabase CLI wraps it). The SHARE
--- lock below makes the "no journal lines" check and the rename atomic against
+-- Explicit transaction block: the CI replay (psql -f per file) and the
+-- Supabase branch runner both execute statements in autocommit, where a bare
+-- LOCK TABLE is refused ("can only be used in transaction blocks"); that is
+-- how 20260908113353 failed on main and blocked the prod queue. The SHARE
+-- lock makes the "no journal lines" check and the rename atomic against
 -- concurrent postings: inserts on journal_entry_lines wait the few
 -- milliseconds this takes, reads are unaffected.
 
+BEGIN;
+
 LOCK TABLE public.journal_entry_lines IN SHARE MODE;
 
 UPDATE public.chart_of_accounts a
@@ -95,3 +100,5 @@ UPDATE public.chart_of_accounts a
           WHERE e.company_id = a.company_id
             AND l.account_number = a.account_number
        );
+
+COMMIT;