feat(workload): switch buildActiveImagesSet to containers index

First consumer migration off the instances table. The image
prune logic now walks the normalized containers.image_ref
column directly — one DB pass against a single table instead
of joining instances against projects to reconstruct the full
"image:tag" string. Demonstrates the consumer-switch pattern
the remaining read sites (proxies, stale scanner, webhook
matcher) will follow.

The legacy `projects []store.Project` parameter is kept on the
function signature for now so call sites don't change in this
commit; the underscore-discard in the body makes it explicit
that it's no longer load-bearing.

internal/api/docker.go

@@ -245,28 +245,24 @@ func sanitizeDockerLogLine(line string) string {
 }
 
 // buildActiveImagesSet returns the set of "image:tag" strings currently used
-// by any instance, computed in a single DB pass instead of N×K queries.
-// Returning an error (rather than swallowing) prevents prune logic from
-// treating a transient DB failure as "nothing is active".
+// by any container, computed in a single DB pass against the normalized
+// containers index. Returning an error (rather than swallowing) prevents
+// prune logic from treating a transient DB failure as "nothing is active".
 func buildActiveImagesSet(st *store.Store, projects []store.Project) (map[string]bool, error) {
-	imageByProject := make(map[string]string, len(projects))
-	for _, p := range projects {
-		imageByProject[p.ID] = p.Image
-	}
-	instances, err := st.ListAllInstances()
+	// `projects` is unused now — kept in the signature for back-compat with
+	// callers that already happen to have the slice. The image_ref column
+	// holds the full "image:tag" string written by the deployer.
+	_ = projects
+	containers, err := st.ListContainers(store.ContainerFilter{})
 	if err != nil {
-		return nil, fmt.Errorf("list instances: %w", err)
+		return nil, fmt.Errorf("list containers: %w", err)
 	}
-	active := make(map[string]bool, len(instances))
-	for _, inst := range instances {
-		if inst.ImageTag == "" {
+	active := make(map[string]bool, len(containers))
+	for _, c := range containers {
+		if c.ImageRef == "" {
 			continue
 		}
-		image := imageByProject[inst.ProjectID]
-		if image == "" {
-			continue
-		}
-		active[image+":"+inst.ImageTag] = true
+		active[c.ImageRef] = true
 	}
 	return active, nil
 }