Put deleted_at in the event-tier indexes so SQLite uses them

GORM adds deleted_at IS NULL to these queries, and SQLite, which has no
table statistics here, preferred the deleted_at index to the new
single-column ones wherever a column is matched against several values
or compared with <. The event log's attempt load, retention's lookups
of expired events and their deliveries, and the queue-depth sampler
still read every live row. Each index now also covers deleted_at,
first in the events index because created_at is compared with <;
events keeps its created_at index for retention's final delete. A test
checks SQLite's plan for each statement as GORM builds it.

Model: opus-5-5

internal/database/model_event.go

@@ -1,16 +1,26 @@
 package database
 
-import "time"
+import (
+	"time"
+
+	"gorm.io/gorm"
+)
 
 // Event represents a captured webhook event
+//
+//nolint:lll // a struct tag cannot wrap
 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"`
+	// CreatedAt and DeletedAt repeat the BaseModel fields only to index
+	// them for retention, which finds events by age. Its lookups carry
+	// GORM's "deleted_at IS NULL" (see DeliveryResult) and compare
+	// created_at with <, so their index has deleted_at first: SQLite
+	// narrows by a < only on the last column it uses. Its final delete
+	// has no deleted_at condition and uses the index on created_at
+	// alone. The other tables keep the unindexed BaseModel created_at.
+	CreatedAt time.Time      `gorm:"index;index:idx_events_deleted_at_created_at,priority:2" json:"createdAt"`
+	DeletedAt gorm.DeletedAt `gorm:"index:idx_events_deleted_at_created_at,priority:1"       json:"deletedAt,omitzero"`
 
 	WebhookID    string `gorm:"type:uuid;not null" json:"webhookId"`
 	EntrypointID string `gorm:"type:uuid;not null" json:"entrypointId"`