feat(deadline-rules): expose concept's canonical event_type per rule

Add paliad.deadline_concept_event_types junction (mig 072) mapping each
deadline_concept to its canonical paliad.event_types row(s). Hydrate
DeadlineRule.ConceptDefaultEventTypeID via one IN query per List call so
/api/deadline-rules carries the autofill hint for the deadline create
form (t-paliad-165 / m/paliad#18).

Seed mapping covers the active concepts driving existing rules — 29
rows across 26 distinct concepts. Concepts without an obvious event_type
counterpart (decision, filing, grant, the DE-only Begründung family)
stay unmapped; auto-fill silently skips them.

internal/models/models.go

@@ -468,6 +468,12 @@ type DeadlineRule struct {
 	AltRuleCode      *string    `db:"alt_rule_code"      json:"alt_rule_code,omitempty"`
 	AnchorAlt        *string    `db:"anchor_alt"         json:"anchor_alt,omitempty"`
 	ConceptID        *uuid.UUID `db:"concept_id"         json:"concept_id,omitempty"`
+	// ConceptDefaultEventTypeID is the canonical paliad.event_types row for
+	// this rule's concept (joined via paliad.deadline_concept_event_types
+	// where is_default = true). Lets the deadline create form auto-populate
+	// the Typ chip when the user picks this rule. Hydrated by the service
+	// layer; not a column. NULL when the concept has no mapped event_type.
+	ConceptDefaultEventTypeID *uuid.UUID `db:"-" json:"concept_default_event_type_id,omitempty"`
 	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"`