feat(t-paliad-265): migration 129 + EventChoiceService (Slice A foundation)

m/paliad#96 — per-event-card optional choices on the Verfahrensablauf
timeline. This commit lands the schema + service layer.

Migration 129:
- paliad.project_event_choices table (project_id, submission_code,
  choice_kind ∈ {appellant, include_ccr, skip}, choice_value) with
  UNIQUE(project_id, submission_code, choice_kind) for idempotent
  re-pick, RLS via paliad.can_see_project.
- paliad.deadline_rules.choices_offered jsonb — opt-in declaration of
  which choice-kinds each rule offers. Seeded for every decision rule
  (appellant), every priority='optional' rule (skip), and the two
  Klageerwiderung rules (upc.inf.cfi.sod + de.inf.lg.erwidg) with
  include_ccr.

Live verification before authoring:
- rule_code is NULL on every decision row → submission_code is the
  join key (matches AnchorOverrides plumbing in fristenrechner.go).
- upc.inf.cfi.sod is the UPC Klageerwiderung, not upc.inf.cfi.def
  (rejected the design doc's first guess; SELECT name ILIKE
  'Klageerwiderung' confirmed).

Go service:
- models.ProjectEventChoice + DeadlineRule.ChoicesOffered.
- EventChoiceService: ListForProject / Upsert (with audit-log row to
  paliad.system_audit_log) / Delete. Pure-helper ToCalcOptionsAddendum
  + per-kind value validation + unit tests.

Design: docs/design-event-card-choices-2026-05-25.md §3 + §6.

internal/services/deadline_rule_service.go

@@ -34,7 +34,8 @@ const ruleColumns = `id, proceeding_type_id, parent_id, submission_code, name, n
 	anchor_alt, concept_id, legal_source, is_spawn, spawn_label, is_active,
 	created_at, updated_at,
 	trigger_event_id, spawn_proceeding_type_id, combine_op, condition_expr,
-	priority, is_court_set, lifecycle_state, draft_of, published_at`
+	priority, is_court_set, lifecycle_state, draft_of, published_at,
+	choices_offered`
 
 const proceedingTypeColumns = `id, code, name, name_en, description, jurisdiction,
 	category, default_color, sort_order, is_active`