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_event.go

@@ -1,9 +1,17 @@
 package database
 
+import "time"
+
 // Event represents a captured webhook event
 type Event struct {
 	BaseModel
 
+	// CreatedAt overrides BaseModel.CreatedAt only to add an index:
+	// retention deletes events by age, so events.created_at is queried
+	// on every sweep. The other tables keep the unindexed BaseModel
+	// field.
+	CreatedAt time.Time `gorm:"index" json:"createdAt"`
+
 	WebhookID    string `gorm:"type:uuid;not null" json:"webhookId"`
 	EntrypointID string `gorm:"type:uuid;not null" json:"entrypointId"`