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

CLAUDE.md

@@ -36,8 +36,9 @@ DATABASE_URL=postgres://... go test -tags integration ./internal/master/store/
 - `internal/master/config`   — env-var loading
 - `internal/master/store`    — Postgres access and migrations
 - `internal/master/hub`      — agent connections: auth, per-connection writer, routing, job reconcile on hello
-- `internal/master/registry` — HEAD-only digest lookup against a registry (go-containerregistry, `DOCKER_CONFIG` credentials)
-- `internal/master/poller`   — every `POLL_INTERVAL`: one lookup per followed `repo:tag`, sets the available version, starts auto jobs
+- `internal/master/registry` — HEAD-only digest lookup and tag listing against a registry (go-containerregistry, `DOCKER_CONFIG` credentials)
+- `internal/master/tagver`   — which tag is a newer version of a followed tag (pure functions)
+- `internal/master/poller`   — every `POLL_INTERVAL`: one lookup per followed `repo:tag`, sets the available version, starts auto jobs; one tag list per repo, sets the newer tag suggestion
 - `internal/master/jobs`     — decides whether a service may be updated and starts a job on the right agent
 - `internal/master/webhook`  — `POST /update`: records the available version, starts jobs for auto services, waits for them
 - `internal/master/web/icons` — embedded Simple Icons path data; slug derived from the image ref; regenerate with `go generate ./internal/master/web/icons`
@@ -80,8 +81,16 @@ DATABASE_URL=postgres://... go test -tags integration ./internal/master/store/
 
 ## Gotchas
 
-- **Registry access is HEAD only** (`internal/master/registry`). Docker Hub counts a manifest
-  GET as a pull; the client never falls back to GET, and a test asserts this.
+- **Registry manifest access is HEAD only** (`internal/master/registry`). Docker Hub counts a
+  manifest GET as a pull; the client never GETs `/manifests/`, and a test asserts this. The
+  only GETs are the `/v2/` ping and `tags/list` (newer-tag detection, once per repo per cycle).
+- **`services.newer_ref` is a suggestion only**: no auto jobs, not even for auto services. The
+  poller writes it only on change; a failed tag list keeps the old value and never sets
+  `last_poll_error`. The UI re-checks it at render time with `tagver.Newer`, so a stale value
+  is hidden until the next cycle; it is also hidden when its repo is not the one the service
+  follows. The suggestion is computed from the service list read at the start of the cycle, so
+  if the service moved on meanwhile, the render-time check hides it. Tags with one numeric component or no digits (`7`, `pg16`,
+  `latest`) never get suggestions.
 - **`image_versions` uses `''` instead of NULL** for a missing tag or digest, so
   `UNIQUE (repo, tag, digest)` deduplicates. The webhook records `(repo, tag, '')`; that
   row is hidden from `RecentVersions` when a row with the same tag and a digest was first