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. 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/export_test.go

@@ -28,6 +28,10 @@ func NewTestRetentionReaper(
 	}
 }
 
+// ExportReapBatchSize exposes how many expired events one retention
+// transaction deletes.
+const ExportReapBatchSize = reapBatchSize
+
 // ExportSweep runs a single retention sweep synchronously for tests.
 func (r *RetentionReaper) ExportSweep(ctx context.Context) {
 	r.sweep(ctx)