fix(m3): uuid secure-context fallback + middle-tier SKU business rules

- frontend: uuid() helper (crypto.randomUUID -> getRandomValues) for plain-HTTP LAN UAT
- slabs.py: empty SKU at create -> NULL (no-SKU-yet is transient, filled in Review)
- review.py: ready-gate enforces REVIEW-45 (SKU 3-24 alnum+hyphen) in the middle tier
- DB CHECK stays as backstop only (referential/structural), per layering principle

backend/slabs.py

@@ -110,8 +110,14 @@ async def create_slab(payload: Dict[str, Any]) -> Dict[str, Any]:
     missing = [f for f in REQUIRED_CREATE_FIELDS if f not in payload]
     if missing:
         return _err("validation", "missing fields", {"fields": missing}, status=400)
+    # Business rule (middle tier, not the DB): an empty/blank SKU at create means
+    # "not set yet" (the owner fills it in Review). Store NULL — the DB CHECK only
+    # governs present SKUs, and NULL is the honest "absent" representation.
+    sku = payload["sku"]
+    if isinstance(sku, str) and sku.strip() == "":
+        payload = {**payload, "sku": None}
     conn = get_connection()
-    if _sku_taken(conn, payload["sku"]):
+    if payload["sku"] is not None and _sku_taken(conn, payload["sku"]):
         return _err("duplicate_sku", "SKU already exists", {"sku": payload["sku"]}, status=409)
     now = _now_iso()
     data = _encode(payload)