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 one row at a time on its own connection inside a
read-only transaction: one snapshot, and no write lock. The handler
holds the rename lock only while it reads the names and opens the file.
A missing archive exports empty and is not created.

Model: opus-5-5

internal/handlers/target_edit_test.go

@@ -37,14 +37,18 @@ const (
 	editAuthHeader   = "Authorization: Bearer " + editBearerSecret
 )
 
-// targetRouter mounts the target create and edit routes on a chi
-// router so the handlers see the URL parameters they read.
+// targetRouter mounts the target create, edit and download routes on
+// a chi router so the handlers see the URL parameters they read.
 func targetRouter(env *sourceTestEnv) *chi.Mux {
 	router := chi.NewRouter()
 	router.Post(
 		"/hook/{sourceID}/targets",
 		env.handlers.HandleTargetCreate(),
 	)
+	router.Get(
+		"/hook/{sourceID}/targets/{targetID}/download",
+		env.handlers.HandleTargetDownload(),
+	)
 	router.Get(
 		"/hook/{sourceID}/targets/{targetID}/edit",
 		env.handlers.HandleTargetEdit(),