Close archive writers when the delivery engine stops (closes #280)

The engine cached archive writers and never closed them at shutdown,
so after a clean stop an archive's rows could sit in its -wal while
the .db held no table. The engine's stop hook now evicts every cached
writer once its workers have returned, the same way deleting a webhook
does, so a clean stop leaves each archive as one file and a late write
is refused. If the workers do not return within the stop budget, the
writers are left open as a kill would leave them: closing would wait
on a write in progress, and a still-running worker would open new
ones.

The README no longer says archives keep their sidecars across a clean
stop.

Model: opus-5-5

internal/delivery/target_database.go

@@ -277,6 +277,24 @@ func (t *databaseTarget) evict(webhookID string) {
 	)
 }
 
+// evictAll evicts every cached archive writer, exactly as evict
+// does for one webhook. The engine calls it at shutdown, once its
+// workers have returned. Closing the last handle on an archive
+// moves the contents of its -wal into the .db and removes the
+// -wal, so a clean stop leaves each archive as a single file.
+func (t *databaseTarget) evictAll() {
+	t.mu.Lock()
+
+	writers := t.writers
+	t.writers = nil
+
+	t.mu.Unlock()
+
+	for _, w := range writers {
+		w.evict()
+	}
+}
+
 // sweepWebhook prunes one webhook's archive of rows older than
 // expiry, without requiring a write. It returns nil (nothing to
 // do) when the archive file does not exist, so a sweep never