feat(t-paliad-183): mig 083 — backfill priority per design §2.3

Phase 3 Slice 2 Step B-2. UPDATE paliad.deadline_rules.priority
from the legacy (is_mandatory, is_optional) pair per DESIGN §2.3
(NOT msg 1746's inverted mapping — head clarified in msg 1750
that design §2.3 is the load-bearing spec).

Mapping:
  T/F (153 rows) → 'mandatory'   (statutory must, ☑ pre-checked)
  T/T (  1 row)  → 'optional'    (RoP.151 — opt-in deadline,
                                  ☐ pre-unchecked per mig 068)
  F/T (  0 rows) → 'recommended' (defensive; no live data)
  F/F ( 18 rows) → 'recommended' (situational filings —
                                  Berufungserwiderung, Replik,
                                  Duplik, R.19 Preliminary
                                  Objection, R.116 EPÜ, etc.)

Why NOT msg 1746's mapping:
  - T/T → 'recommended' would PRE-CHECK RoP.151 in the save modal
    and auto-create a Kostenentscheidung deadline the user didn't
    ask for. That's the regression we'd ship.
  - F/F → 'informational' would render 18 real filing deadlines
    NEVER-SAVEABLE per design §2.3 ("informational … NEVER saves
    as a deadline"). They'd disappear from save flows entirely.

T/F branch is intentionally skipped — mig 078 already defaults
priority='mandatory', so all 153 T/F rows are already correct.
Writing 153 needless audit rows would dilute the backfill trail.

Audit-reason cites design §2.3 — that's the persistent rationale
captured in paliad.deadline_rule_audit. Migration enforces NOT NULL
post-run via a DO block that RAISE EXCEPTION on stragglers.

internal/db/migrations/083_backfill_priority.down.sql

@@ -0,0 +1,17 @@
+-- t-paliad-183 down — reverts the priority flips written by
+-- 083_backfill_priority.up.sql.
+--
+-- "Revert" here means: restore the post-Slice-1 column default
+-- ('mandatory' on every row). Mig 078 created the column with that
+-- default; post-Slice-1 every row was 'mandatory' regardless of its
+-- (is_mandatory, is_optional) pair. Resetting to 'mandatory' is
+-- therefore equivalent to "undo the backfill".
+
+SELECT set_config(
+    'paliad.audit_reason',
+    'rollback 083: reset priority to mig 078 default (mandatory)',
+    true);
+
+UPDATE paliad.deadline_rules
+   SET priority = 'mandatory'
+ WHERE priority <> 'mandatory';