refactor(blog,marketing-pages): wire coverage helper + declare manifest bands

Migrates blog and marketing-pages to the same pattern auth landed in
f7baa8b: vitest config consumes vitestThresholdsFromBands(
DEFAULT_COVERAGE_BANDS) instead of the duplicated hand-written
thresholds block, and each manifest declares its coverage section
explicitly.

Verified semantically identical to the previous hand-written
thresholds — the same numbers come out the other end of the helper.
No new regressions:
  - blog: 89 tests, 96.33% overall, all bands green
  - marketing-pages: 67 tests, 95.28% overall — controllers/ shows a
    real 93.54% lines / 90.9% branches gap that has been there since
    before this refactor (the previous hand-written threshold was the
    same 100%/95%). This is one of the L0 unification work items
    listed in the PRD's findings; capturing here as the third feature
    with real test gaps (navigation, media, marketing-pages).

Three of five features now drive their vitest thresholds from the
manifest helper: auth ✓, blog ✓, marketing-pages ✓. Navigation and
media stay on the legacy config until L0 unification closes their
test gaps (touching them would expose the same failures and add
nothing).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/blog/src/feature.manifest.ts

@@ -28,6 +28,25 @@ export const blogManifest = defineFeature({
   },
   realtimeChannels: [],
   jobs: [],
+  coverage: {
+    bands: {
+      baseline: { statements: 80, branches: 75, functions: 80, lines: 80 },
+      entities: { statements: 100, branches: 100, functions: 100, lines: 100 },
+      "use-cases": {
+        statements: 100,
+        branches: 95,
+        functions: 100,
+        lines: 100,
+      },
+      controllers: {
+        statements: 100,
+        branches: 95,
+        functions: 100,
+        lines: 100,
+      },
+    },
+    mutationTargets: ["entities", "use-cases"],
+  },
 } as const);
 
 export type BlogManifest = typeof blogManifest;