feat(deadline-rules/is-optional): conditional rules opt-in via save modal

m's 2026-05-08 batch Item 2: some rules don't always apply per-instance.
Antrag auf Kostenentscheidung (RoP.151) only fires when a party files
for it; some appeal-related deadlines depend on specific facts. Today
they render in the timeline as if always applicable; the save-to-
project modal pre-checks them so the user has to remember to uncheck.

New paliad.deadline_rules.is_optional bool flag (default false). Threads
through the Go model, ruleColumns SELECT, UIDeadline JSON, and the
frontend save modal:

  - Migration 066 adds the column + comment + a starter UPDATE that
    flips RoP.151 to is_optional=true. m can flip more via SQL as he
    reviews the rule library — distinct from is_mandatory, which is
    about statutory strictness once the rule applies (an "auf Antrag"
    rule can be is_mandatory=true once requested).
  - Save modal: optional rows pre-uncheck (the user opts in) and a
    small "auf Antrag" / "on request" pill renders in the meta line.
    Court-determined rows still pre-uncheck via the existing disabled
    path; isOptional doesn't override that.

Migration applied to live Supabase; tracker at v66.

Refs m/paliad#15 (m's 2026-05-08 18:21 follow-up batch Item 2).

internal/services/deadline_rule_service.go

@@ -25,7 +25,8 @@ const ruleColumns = `id, proceeding_type_id, parent_id, code, name, name_en,
 	description, primary_party, event_type, is_mandatory, duration_value,
 	duration_unit, timing, rule_code, deadline_notes, deadline_notes_en, sequence_order,
 	condition_rule_id, condition_flag, alt_duration_value, alt_duration_unit, alt_rule_code,
-	anchor_alt, concept_id, legal_source, is_spawn, spawn_label, is_active, created_at, updated_at`
+	anchor_alt, concept_id, legal_source, is_spawn, spawn_label, is_optional, is_active,
+	created_at, updated_at`
 
 const proceedingTypeColumns = `id, code, name, name_en, description, jurisdiction,
 	category, default_color, sort_order, is_active`