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/server/routes.go

@@ -312,6 +312,10 @@ func (s *Server) setupSourceRoutes() {
 			"/targets/{targetID}/edit",
 			s.h.HandleTargetEditSubmit(),
 		)
+		r.Get(
+			"/targets/{targetID}/download",
+			s.h.HandleTargetDownload(),
+		)
 		r.Post(
 			"/targets/{targetID}/delete",
 			s.h.HandleTargetDelete(),