feat(master): suggest newer version tags for third-party images (#11) (#31)

internal/master/store/migrations/0007_newer_tag.sql

@@ -0,0 +1,9 @@
+-- Newer version tag of the followed tag, found in the registry's tag list.
+-- A suggestion only: nothing deploys it automatically.
+ALTER TABLE services ADD COLUMN newer_ref text NOT NULL DEFAULT '';
+ALTER TABLE services ADD COLUMN newer_at  timestamptz;
+
+-- A suggested tag is recorded in image_versions with source 'tags'.
+ALTER TABLE image_versions DROP CONSTRAINT image_versions_source_check;
+ALTER TABLE image_versions ADD CONSTRAINT image_versions_source_check
+    CHECK (source IN ('webhook', 'poll', 'running', 'tags'));