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_result.go

@@ -1,10 +1,21 @@
 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"   json:"deliveryId"`
+	// 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"`