fix(mig 099): add missing audit_reason wrapper

Mig 099 (drop_with_po_flag) crash-looped paliad.de prod immediately
after deploy: the mig 079 trigger on paliad.deadline_rules raises
EXCEPTION 'audit reason required' on UPDATE when paliad.audit_reason
is unset. Original file (fermi, t-paliad-207) only had the UPDATE,
no set_config wrapper.

Patch: prepend the standard 'SELECT set_config(paliad.audit_reason,
...)' at the top so the trigger sees the reason. Same shape as every
other migration that mutates deadline_rules.

Manual recovery already applied via head MCP — UPDATE'd the 2 rows
with audit_reason set, marked tracker version=99 dirty=false,
force-restarted the container which booted clean. This commit aligns
the in-repo file with the recovered prod state. Idempotent: the
WHERE clause matches only rows that still carry with_po, so re-apply
is a no-op.

internal/db/migrations/099_drop_with_po_flag.up.sql

@@ -14,6 +14,16 @@
 -- Two rows updated; pinned by proceeding code so this stays correct
 -- after any rule-id reshuffle. Idempotent: the WHERE clause matches
 -- the live shape, so re-apply is a no-op.
+--
+-- audit_reason set_config required at the top — the mig 079 trigger
+-- on paliad.deadline_rules raises EXCEPTION 'audit reason required'
+-- on any UPDATE without it. Original mig 099 author missed this and
+-- crash-looped paliad prod; this is the recovery patch.
+
+SELECT set_config(
+    'paliad.audit_reason',
+    'mig 099: drop with_po condition_expr on the two RoP.019.1 rows — m''s call 2026-05-18 (t-paliad-207 interactive session), R.19 Einspruch is always-available not flag-gated',
+    true);
 
 UPDATE paliad.deadline_rules dr
    SET condition_expr = NULL