Add a statistics pane to the webhook page (closes #368)

Each webhook's event database keeps running totals: one row for its
events, and one row per target for that target's deliveries, delivered
and failed, each with what retention removed. Every write to them
shares the transaction of the rows it counts. Deliveries get a
finished_at column; it and target_id end the status index, so each
target's deliveries finished in a window come from one index-range
query grouped by target. Retention deletes 1000 expired events per
transaction, pausing 200 ms between them so other writers get in. The
pane is its own template, its figures in tables.

The schema changes in place with nothing back-filled, so an existing
database must be recreated.

Model: opus-5-5

internal/database/model_delivery.go

@@ -1,6 +1,10 @@
 package database
 
-import "gorm.io/gorm"
+import (
+	"time"
+
+	"gorm.io/gorm"
+)
 
 // DeliveryStatus represents the status of a delivery
 type DeliveryStatus string
@@ -37,7 +41,7 @@ type Delivery struct {
 	BaseModel
 
 	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"`
+	TargetID string         `gorm:"type:uuid;not null;index:idx_deliveries_status,priority:4"         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
@@ -45,6 +49,13 @@ type Delivery struct {
 	// gives.
 	DeletedAt gorm.DeletedAt `gorm:"index:idx_deliveries_event_id,priority:2;index:idx_deliveries_status,priority:2" json:"deletedAt,omitzero"`
 
+	// FinishedAt is when the delivery became delivered or failed, and
+	// nil while it is pending or retrying. It and then TargetID end the
+	// status index, so the webhook page counts each target's deliveries
+	// that finished in a recent window by reading just that window from
+	// the index.
+	FinishedAt *time.Time `gorm:"index:idx_deliveries_status,priority:3" json:"finishedAt,omitempty"`
+
 	// Relations
 	Event           Event            `json:"event,omitzero"`
 	Target          Target           `json:"target,omitzero"`