Index the event-tier columns the sweeps, event log and retention scan (closes #314)

The per-webhook tables declared no secondary indexes, so the recovery
and sweep queries (by delivery status, every minute), the event log
(deliveries by event, results by delivery) and retention (events by
age) each scanned a whole table. Add indexes through GORM model tags so
AutoMigrate creates them on a fresh and on an existing per-webhook
database. events.created_at is indexed by overriding the embedded
BaseModel field on Event alone, leaving the other tables' created_at
unindexed. A test drops the indexes from an opened database, reopens it,
and asserts the open recreated them. The README Data Model section lists
the indexes.

Model: opus-4-8

internal/database/model_delivery.go

@@ -32,9 +32,9 @@ func (s DeliveryStatus) Terminal() bool {
 type Delivery struct {
 	BaseModel
 
-	EventID  string         `gorm:"type:uuid;not null"         json:"eventId"`
-	TargetID string         `gorm:"type:uuid;not null"         json:"targetId"`
-	Status   DeliveryStatus `gorm:"not null;default:'pending'" json:"status"`
+	EventID  string         `gorm:"type:uuid;not null;index"         json:"eventId"`
+	TargetID string         `gorm:"type:uuid;not null"               json:"targetId"`
+	Status   DeliveryStatus `gorm:"not null;default:'pending';index" json:"status"`
 
 	// Relations
 	Event           Event            `json:"event,omitzero"`