fix(auto-update): label same-tag rebuilds as 'Rebuild available' instead of '10.11 -> 10.11' (#766)

When a registry pushes a new build of an image at the same tag (digest
changes, tag does not), the preview service set next_tag to the same
string as current_tag and the readiness view rendered '10.11 -> 10.11',
which reads as a UI bug.

Add an update_kind field to UpdatePreviewSummary that distinguishes:
- 'tag'    - at least one image has a strictly newer tag
- 'digest' - the only updates available are same-tag rebuilds
- 'none'   - nothing to apply

The frontend now branches on update_kind and renders 'Rebuild available'
next to the current tag for the digest case, leaving the version-arrow
diff for genuine tag bumps.

Three new buildSummary cases lock in the kind classification.

backend/src/__tests__/update-preview-service.test.ts

@@ -224,4 +224,23 @@ describe('buildSummary', () => {
         expect(preview.summary.semver_bump).toBe('major');
         expect(preview.summary.blocked).toBe(true);
     });
+
+    it('reports update_kind="tag" when at least one image has a strictly newer tag', () => {
+        const images = [
+            baseImage({ service: 'web', has_update: true, semver_bump: 'patch', next_tag: '1.0.1', current_tag: '1.0.0' }),
+        ];
+        expect(buildSummary('stacky', images).summary.update_kind).toBe('tag');
+    });
+
+    it('reports update_kind="digest" when only same-tag rebuilds are available', () => {
+        const images = [
+            baseImage({ service: 'web', has_update: true, semver_bump: 'patch', next_tag: '10.11', current_tag: '10.11', image: 'redis:10.11' }),
+        ];
+        expect(buildSummary('stacky', images).summary.update_kind).toBe('digest');
+    });
+
+    it('reports update_kind="none" when nothing has an update', () => {
+        const images = [baseImage({ service: 'clean', has_update: false })];
+        expect(buildSummary('stacky', images).summary.update_kind).toBe('none');
+    });
 });

backend/src/services/UpdatePreviewService.ts

@@ -24,12 +24,21 @@ export interface UpdatePreviewImage {
     semver_bump: SemverBump;
 }
 
+export type UpdateKind = 'tag' | 'digest' | 'none';
+
 export interface UpdatePreviewSummary {
     has_update: boolean;
     primary_image: string | null;
     current_tag: string | null;
     next_tag: string | null;
     semver_bump: SemverBump;
+    /**
+     * Distinguishes a "new tag is available" update from a "same tag, new
+     * digest" rebuild. The UI renders the rebuild case differently because
+     * showing "10.11 -> 10.11" reads as a bug even though it is technically
+     * accurate (the tag did not change, only the immutable digest behind it).
+     */
+    update_kind: UpdateKind;
     blocked: boolean;
     blocked_reason: string | null;
 }
@@ -208,6 +217,14 @@ export function buildSummary(stackName: string, images: UpdatePreviewImage[]): U
         'none',
     );
     const blocked = overallBump === 'major';
+    // 'tag' means at least one image has a strictly newer tag; 'digest' means
+    // the only updates available are same-tag rebuilds (digest changed); 'none'
+    // means there is nothing to apply.
+    const updateKind: UpdateKind = !hasUpdate
+        ? 'none'
+        : updated.some(i => i.next_tag !== null && i.next_tag !== i.current_tag)
+            ? 'tag'
+            : 'digest';
     return {
         stack_name: stackName,
         images,
@@ -217,6 +234,7 @@ export function buildSummary(stackName: string, images: UpdatePreviewImage[]): U
             current_tag: primary ? primary.current_tag : null,
             next_tag: primary ? primary.next_tag : null,
             semver_bump: overallBump,
+            update_kind: updateKind,
             blocked,
             blocked_reason: blocked ? 'Major version jumps require human review before applying.' : null,
         },

frontend/src/components/AutoUpdateReadinessView.tsx

@@ -19,6 +19,8 @@ interface UpdatePreviewImage {
   semver_bump: SemverBump;
 }
 
+type UpdateKind = 'tag' | 'digest' | 'none';
+
 interface UpdatePreview {
   stack_name: string;
   images: UpdatePreviewImage[];
@@ -28,6 +30,7 @@ interface UpdatePreview {
     current_tag: string | null;
     next_tag: string | null;
     semver_bump: SemverBump;
+    update_kind: UpdateKind;
     blocked: boolean;
     blocked_reason: string | null;
   };
@@ -160,10 +163,19 @@ function StackReadinessCard({
           const blockedReason = p.summary.blocked_reason;
           return (
             <>
-              <VersionDiff
-                current={p.summary.current_tag}
-                next={p.summary.next_tag}
-              />
+              {p.summary.update_kind === 'digest' ? (
+                <div className="flex items-baseline gap-2 font-mono text-sm">
+                  <span className="text-stat-subtitle">{p.summary.current_tag}</span>
+                  <span className="text-brand text-[11px] uppercase tracking-[0.16em]">
+                    Rebuild available
+                  </span>
+                </div>
+              ) : (
+                <VersionDiff
+                  current={p.summary.current_tag}
+                  next={p.summary.next_tag}
+                />
+              )}
 
               <div className="flex items-center gap-1.5 font-mono text-[11px] text-stat-subtitle/80">
                 <span>{p.summary.primary_image ?? '-'}</span>