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

The per-webhook event databases had no secondary indexes, so startup
recovery, the retry and pending sweeps, the queue-depth sampler, the
event log and retention each read whole tables. Indexes declared in
the GORM model tags now serve them, and AutoMigrate adds them to new
and existing databases alike.

Each index also covers deleted_at: GORM adds deleted_at IS NULL to
these queries, and SQLite, with no table statistics, otherwise
prefers the existing deleted_at index. A test checks SQLite's plan
for each statement as GORM builds it.

Rule suppressed: lll on the three event-tier model structs, whose
struct tags cannot wrap.
The resubmitted_from_id scan is left to
#325.

Model: opus-4-8 (implementation); opus-5-5 (rework)

internal/database/model_delivery.go

@@ -1,5 +1,7 @@
 package database
 
+import "gorm.io/gorm"
+
 // DeliveryStatus represents the status of a delivery
 type DeliveryStatus string
 
@@ -29,12 +31,19 @@ func (s DeliveryStatus) Terminal() bool {
 }
 
 // Delivery represents a delivery attempt for an event to a target
+//
+//nolint:lll // a struct tag cannot wrap
 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:idx_deliveries_event_id,priority:1"       json:"eventId"`
+	TargetID string         `gorm:"type:uuid;not null"                                                json:"targetId"`
+	Status   DeliveryStatus `gorm:"not null;default:'pending';index:idx_deliveries_status,priority:1" json:"status"`
+
+	// DeletedAt repeats the BaseModel field only to be the second column
+	// of the event_id and status indexes, for the reason DeliveryResult
+	// gives.
+	DeletedAt gorm.DeletedAt `gorm:"index:idx_deliveries_event_id,priority:2;index:idx_deliveries_status,priority:2" json:"deletedAt,omitzero"`
 
 	// Relations
 	Event           Event            `json:"event,omitzero"`