feat(store): available ref, update mode and services-by-repo for the webhook (#8)

internal/master/store/migrations/0003_webhook.sql

@@ -0,0 +1,6 @@
+ALTER TABLE services ADD COLUMN available_ref    text NOT NULL DEFAULT '';
+ALTER TABLE services ADD COLUMN available_source text NOT NULL DEFAULT ''
+    CHECK (available_source IN ('', 'webhook', 'poll'));
+ALTER TABLE services ADD COLUMN available_at     timestamptz;
+
+CREATE INDEX services_by_image_repo ON services (image_repo) WHERE removed_at IS NULL;