feat(backend): scan-product accuracy 66.2% -> 79.7% + frozen validation benchmark

Accuracy work on the 79-image product-scan validation set (user goal: 90%):
- classify_ocr_server.py: 0/90/180/270-degree expiry-date search (stops at
  first hit, 0-degree fallback); classification decoupled onto the upright
  image (rotated frames regressed DINOv2 -6pts until this); cross-line date
  stitching; tiled full-res OCR pass (defeats the 4000px downscale that
  killed small inkjet dates); VL-pipeline expiry fallback with
  keyword-anchored anti-hallucination guard; VL text lines merged into
  text_lines + VL SKU retry. Visualization endpoints removed entirely
  (Visual/Spotting grids - unused by frontend, 3x per-scan GPU cost).
- product-scan.ts: coverage-normalized OCR-evidence re-ranking of DINOv2
  top-K (tuned offline: +8/-0 on top-1 misses), re-ranked class mapped to
  sku_master by SKU prefix; classifier timeout 90s->240s for fallback paths.
- Frozen benchmark: product-test-images-fixed/ (79 renamed images) +
  freeze/seed/build-undetected/capture/experiment scripts; labels trimmed to
  the 79 validation entries (training rows kept in .bak-with-training);
  5 TRAINED-ON SKUs replaced with fresh held-out photos.
- manual-label-scan page: shows last batch-test AI prediction under every
  field by default (new /api/product-scan-results); serves the fixed folder;
  fixed total hydration failure via allowedDevOrigins 127.0.0.1.
- Measured (all-79, zero failures): sku/name 87.3%, expiry 64.6%, overall
  79.7%. Tiles/VL-evidence/VL-SKU deployed but not yet batch-measured.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Gr6HH7JrdsXX8AARejQboM

backend/scripts/freeze-validation-set.mjs

@@ -0,0 +1,47 @@
+// One-off script: freeze the current 74-image product-scan Validation Set into
+// a dedicated, stable folder (sources/product-test-images-fixed/) so re-running
+// the accuracy harness always scores the exact same images, independent of
+// whatever new photos get dropped into the live-intake folder
+// (sources/product-test-images/, still fed by the /manual-label-scan page).
+// Renames each image "<index> <no_sku>.<ext>" (index = its stable position,
+// 1-based) and updates product_manual_labels.json's flat-filename entries to
+// match. Run once from backend/: node scripts/freeze-validation-set.mjs
+import fs from "fs";
+import path from "path";
+
+const LIVE_DIR = path.join("sources", "product-test-images");
+const FIXED_DIR = path.join("sources", "product-test-images-fixed");
+const LABELS_PATH = path.join("sources", "product_manual_labels.json");
+
+const labels = JSON.parse(fs.readFileSync(LABELS_PATH, "utf8"));
+const flatEntries = labels.filter((l) => !l.filename.includes("/"));
+
+if (!fs.existsSync(FIXED_DIR)) fs.mkdirSync(FIXED_DIR, { recursive: true });
+
+// Resolve by SKU prefix (not entry.filename directly) so this script is
+// idempotent/rerunnable even after a previous run already renamed
+// entry.filename to "<index> <sku>.<ext>" — the live-intake folder always
+// keeps its original "<sku> <product>__<camera-filename>.<ext>" names.
+const liveFiles = fs.readdirSync(LIVE_DIR);
+function findSourceFile(no_sku) {
+  const match = liveFiles.find((f) => f.startsWith(`${no_sku} `) || f.startsWith(`${no_sku}__`));
+  if (!match) return null;
+  return path.join(LIVE_DIR, match);
+}
+
+let copied = 0;
+flatEntries.forEach((entry, i) => {
+  const index = i + 1;
+  const srcPath = findSourceFile(entry.no_sku);
+  if (!srcPath) {
+    throw new Error(`Missing source image for ${entry.no_sku} in ${LIVE_DIR}`);
+  }
+  const ext = path.extname(srcPath);
+  const newFilename = `${index} ${entry.no_sku}${ext}`;
+  fs.copyFileSync(srcPath, path.join(FIXED_DIR, newFilename));
+  entry.filename = newFilename;
+  copied++;
+});
+
+fs.writeFileSync(LABELS_PATH, JSON.stringify(labels, null, 2), "utf8");
+console.log(`Copied ${copied} images into ${FIXED_DIR} and updated ${LABELS_PATH}.`);