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_delivery_result.go

@@ -1,14 +1,25 @@
 package database
 
+import "gorm.io/gorm"
+
 // DeliveryResult represents the result of a delivery attempt
+//
+//nolint:lll // a struct tag cannot wrap
 type DeliveryResult struct {
 	BaseModel
 
-	DeliveryID   string `gorm:"type:uuid;not null;index" json:"deliveryId"`
-	AttemptNum   int    `gorm:"not null"                 json:"attemptNum"`
+	// DeliveryID and DeletedAt make up one index, in that order.
+	// DeletedAt repeats the BaseModel field only to join it: GORM adds
+	// "deleted_at IS NULL" to almost every query, and where a column is
+	// matched against several values SQLite otherwise reads through the
+	// deleted_at index, which every live row matches.
+	DeliveryID string         `gorm:"type:uuid;not null;index:idx_delivery_results_delivery_id,priority:1" json:"deliveryId"`
+	DeletedAt  gorm.DeletedAt `gorm:"index:idx_delivery_results_delivery_id,priority:2"                    json:"deletedAt,omitzero"`
+
+	AttemptNum   int    `gorm:"not null"             json:"attemptNum"`
 	Success      bool   `json:"success"`
 	StatusCode   int    `json:"statusCode,omitempty"`
-	ResponseBody string `gorm:"type:text"                json:"responseBody,omitempty"`
+	ResponseBody string `gorm:"type:text"            json:"responseBody,omitempty"`
 	Error        string `json:"error,omitempty"`
 	Duration     int64  `json:"durationMs"` // Duration in milliseconds