Add a Download button that exports a database target's archive as gzipped JSON (closes #374)

Each database target on the webhook page links to
/hook/ID/targets/TARGETID/download, which streams the target's archive
as archive-WEBHOOK-TARGET-YYYYMMDDTHHMMSSZ.json.gz: the webhook, the
target, exported_at, and archived_events, one object per row keyed by
column, a body that is not valid UTF-8 in base64 with body_encoding.
The export reads through one cursor inside a read-only transaction:
one snapshot, no write lock. A download runs for as long as the client
keeps reading, and one that fails after it has started aborts the
connection. The request limit answers 504 only when the handler has
not started its response, so a long download is logged as its 200.

Model: opus-5-5

internal/delivery/target_database.go

@@ -3,7 +3,6 @@ package delivery
 import (
 	"context"
 	"fmt"
-	"path/filepath"
 	"strings"
 	"sync"
 	"time"
@@ -277,10 +276,9 @@ func (t *databaseTarget) releaseSweepWriter(
 }
 
 // newWriter builds the writer for a database target's archive. The
-// file lives beside the webhook's event database in the data
-// directory and is named for the webhook and the target as the main
-// database has them now; from then on only rename changes the name
-// the writer uses. It does not touch the archive file.
+// file is the one ArchivePath gives for the webhook and the target as
+// the main database names them now; from then on only rename changes
+// the name the writer uses. It does not touch the archive file.
 func (t *databaseTarget) newWriter(
 	targetID string,
 ) (*archiveWriter, error) {
@@ -299,12 +297,10 @@ func (t *databaseTarget) newWriter(
 		)
 	}
 
-	dir := filepath.Dir(t.eng.dbManager.DBPath(target.WebhookID))
-	name := ArchiveFileName(
-		target.Webhook.Name, target.Name, target.ID,
+	w := newArchiveWriter(
+		ArchivePath(t.eng.dbManager, &target.Webhook, &target),
+		t.eng.log,
 	)
-
-	w := newArchiveWriter(filepath.Join(dir, name), t.eng.log)
 	w.webhookID = target.WebhookID
 
 	return w, nil