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/models/models.go

@@ -413,6 +413,11 @@ type DeadlineRule struct {
 	LegalSource      *string    `db:"legal_source"       json:"legal_source,omitempty"`
 	IsSpawn          bool       `db:"is_spawn"           json:"is_spawn"`
 	SpawnLabel       *string    `db:"spawn_label"        json:"spawn_label,omitempty"`
+	// IsOptional flags a rule whose deadline is conditional on a user
+	// act (e.g. RoP.151 cost-decision request — only fires when a
+	// party files for it). Save-modal pre-unchecks optional rows; the
+	// timeline still renders them so the user knows what could apply.
+	IsOptional       bool       `db:"is_optional"        json:"is_optional"`
 	IsActive         bool       `db:"is_active"          json:"is_active"`
 	CreatedAt        time.Time  `db:"created_at"         json:"created_at"`
 	UpdatedAt        time.Time  `db:"updated_at"         json:"updated_at"`