docker: an update clears "update available" without a page reload

The registry's answer was cached and served as-is, so the stack view and the
machine-wide list kept calling an image behind after an update had pulled it.
The cache now keeps the digest the registry named, and an image marked behind
is re-checked locally on every read: once this machine holds that digest it
is current, at no registry cost. The page re-reads the machine-wide answer
after a stack task, a queued update, or a schedule run.

Also: stack keys can no longer collide with the machine key, and the dead
"/"+stack forget in the schedule runner is gone.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/stacks.go

@@ -744,7 +744,6 @@ func (s *Server) stackAction(w http.ResponseWriter, r *http.Request) {
 			_ = exec.CommandContext(ctx, dockerTool(), "exec", via, "pkill", "-TERM", "-f", pattern).Run()
 		})
 	}
-	stackUpdatesForget(st.Name)
 	log.Printf("stack %s: %s started as task %s (from %s)", st.Name, r.PathValue("action"), t.ID, clientIP(r))
 	jsonOK(w, 202, t.Snapshot(false))
 }
@@ -755,6 +754,7 @@ type imageCheck struct {
 	Image  string `json:"image"`
 	State  string `json:"state"` // current | update | unknown
 	Detail string `json:"detail,omitempty"`
+	remote string // the digest the registry named, so a later pull can be recognised without asking again
 }
 
 type updAnswer struct {
@@ -767,7 +767,41 @@ var (
 	updCache = map[string]updAnswer{}
 )
 
-func stackUpdatesForget(key string) { updMu.Lock(); delete(updCache, key); updMu.Unlock() }
+func stackUpdatesForget(name string) { updMu.Lock(); delete(updCache, "stack:"+name); updMu.Unlock() }
+
+// localDigests is what this machine holds for an image: the registry digests it was pulled as.
+func localDigests(ctx context.Context, img string) []string {
+	out, err := dockerRun(ctx, "image", "inspect", "-f", "{{json .RepoDigests}}", img)
+	var have []string
+	if err != nil || json.Unmarshal(bytes.TrimSpace(out), &have) != nil {
+		return nil
+	}
+	return have
+}
+
+func holdsDigest(have []string, digest string) bool {
+	for _, h := range have {
+		if strings.HasSuffix(h, "@"+digest) {
+			return true
+		}
+	}
+	return false
+}
+
+// settled is a cached answer brought up to date with what is here now. The registry's side
+// is kept for six hours, but this machine's side changes the moment an update pulls the
+// newer image — so an image marked behind is looked at again locally, which costs no
+// registry request, and is current once it holds the digest the registry named.
+func settled(ctx context.Context, res []imageCheck) []imageCheck {
+	out := make([]imageCheck, len(res))
+	copy(out, res)
+	for i, ic := range out {
+		if ic.State == "update" && ic.remote != "" && holdsDigest(localDigests(ctx, ic.Image), ic.remote) {
+			out[i].State = "current"
+		}
+	}
+	return out
+}
 
 // checkImages asks the registry whether each image is still the one we hold. Each image is
 // one registry request, which Docker Hub counts — so this is only ever called when asked for.
@@ -775,9 +809,8 @@ func checkImages(ctx context.Context, images []string) []imageCheck {
 	res := []imageCheck{}
 	for _, img := range images {
 		ic := imageCheck{Image: img, State: "unknown"}
-		out, err := dockerRun(ctx, "image", "inspect", "-f", "{{json .RepoDigests}}", img)
-		var have []string
-		if err != nil || json.Unmarshal(bytes.TrimSpace(out), &have) != nil || len(have) == 0 {
+		have := localDigests(ctx, img)
+		if len(have) == 0 {
 			ic.Detail = "built or loaded here, not pulled from a registry"
 			res = append(res, ic)
 			continue
@@ -795,11 +828,9 @@ func checkImages(ctx context.Context, images []string) []imageCheck {
 			res = append(res, ic)
 			continue
 		}
-		ic.State = "update"
-		for _, h := range have {
-			if strings.HasSuffix(h, "@"+digest) {
-				ic.State = "current"
-			}
+		ic.State, ic.remote = "update", digest
+		if holdsDigest(have, digest) {
+			ic.State = "current"
 		}
 		res = append(res, ic)
 	}
@@ -833,7 +864,7 @@ func (s *Server) machineUpdates(w http.ResponseWriter, r *http.Request) {
 	c, ok := updCache[key]
 	updMu.Unlock()
 	if ok && time.Since(c.at) < 6*time.Hour && !refresh {
-		jsonOK(w, 200, map[string]any{"images": c.res, "checked": c.at.UnixMilli(), "stacks": byImage})
+		jsonOK(w, 200, map[string]any{"images": settled(ctx, c.res), "checked": c.at.UnixMilli(), "stacks": byImage})
 		return
 	}
 	if !refresh {
@@ -863,13 +894,13 @@ func (s *Server) stackUpdates(w http.ResponseWriter, r *http.Request) {
 		jsonErr(w, 404, err.Error())
 		return
 	}
-	key := st.Name
+	key := "stack:" + st.Name
 	updMu.Lock()
 	c, ok := updCache[key]
 	updMu.Unlock()
 	refresh := r.URL.Query().Get("refresh") == "1"
 	if ok && time.Since(c.at) < 6*time.Hour && !refresh {
-		jsonOK(w, 200, map[string]any{"images": c.res, "checked": c.at.UnixMilli()})
+		jsonOK(w, 200, map[string]any{"images": settled(ctx, c.res), "checked": c.at.UnixMilli()})
 		return
 	}
 	if !refresh {