Show each entrypoint's last event and event count on the webhook page (closes #393)

The entrypoint list showed no sign of whether anything uses an entrypoint, so an operator with several could not tell which senders are live before deactivating or deleting one. Each entrypoint now shows when its last event arrived, relative with the UTC time on hover, or "never", and how many events arrived through it within the webhook's retention. The last-event time comes from a new entrypoint_totals row written in the transaction that stores the event and left by retention, so a sender quieter than the retention period does not read "never". The count is one grouped query over a new index. Resubmitted copies count in neither. Pre-1.0: schema changed in place.

Model: opus-5-5

internal/database/model_event.go

@@ -20,12 +20,15 @@ type Event struct {
 	// has no deleted_at condition and uses the index on created_at
 	// alone. The other tables keep the unindexed BaseModel created_at.
 	// DeletedAt is also the second column of the resubmitted_from_id
-	// index, for the reason DeliveryResult gives.
-	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;index:idx_events_resubmitted_from_id,priority:2" json:"deletedAt,omitzero"`
+	// index, for the reason DeliveryResult gives. The entrypoint_id
+	// index, for the webhook page's entrypoint list, has it second too,
+	// resubmitted_from_id third, and created_at last, which the list
+	// compares with a range.
+	CreatedAt time.Time      `gorm:"index;index:idx_events_deleted_at_created_at,priority:2;index:idx_events_entrypoint_id,priority:4"                                           json:"createdAt"`
+	DeletedAt gorm.DeletedAt `gorm:"index:idx_events_deleted_at_created_at,priority:1;index:idx_events_resubmitted_from_id,priority:2;index:idx_events_entrypoint_id,priority:2" json:"deletedAt,omitzero"`
 
-	WebhookID    string `gorm:"type:uuid;not null" json:"webhookId"`
-	EntrypointID string `gorm:"type:uuid;not null" json:"entrypointId"`
+	WebhookID    string `gorm:"type:uuid;not null"                                           json:"webhookId"`
+	EntrypointID string `gorm:"type:uuid;not null;index:idx_events_entrypoint_id,priority:1" json:"entrypointId"`
 
 	// Request data
 	Method      string `gorm:"not null"    json:"method"`
@@ -44,7 +47,7 @@ type Event struct {
 	// existed. It is not a foreign key: the source event can be
 	// reaped by retention while its copies remain, and the id is
 	// kept as the record of where the copy came from either way.
-	ResubmittedFromID *string `gorm:"type:uuid;index:idx_events_resubmitted_from_id,priority:1" json:"resubmittedFromId,omitempty"`
+	ResubmittedFromID *string `gorm:"type:uuid;index:idx_events_resubmitted_from_id,priority:1;index:idx_events_entrypoint_id,priority:3" json:"resubmittedFromId,omitempty"`
 
 	// Relations. No model marshals the record it belongs to, so
 	// Webhook and Entrypoint are left out of the JSON.