Ask instead of guessing when a label fits several products

`"britannia"` is a substring of all 258 Britannia product names, and
textScore returned 0.95 for any product whose name contained the label.
So every one of them tied, the tie broke alphabetically, and the customer
was shown one arbitrary biscuit with "confidence": 0.95 and a price. Lens
hands back a bare wordmark often — it is usually the biggest thing printed
on a packet — so this was the common case, not an edge one. Found via the
example request in the mobile team's own proposal.

Scoring now asks both questions. A hit carries `score` (ranks) and `text`
(how specifically the label names THIS product: the harmonic mean of how
much of the label the product explains and how much of the product's name
the label explains, pack sizes dropped from both sides). A brand name
scores its products ~0.33 equally instead of 0.95 arbitrarily. The
"vector and text agree" bonus is now proportional to the text score, so a
weak match can no longer inflate a whole brand.

isAmbiguous reads that: the leader is a guess if anything is level with it
(margin) or if the label names no one product (specificity), and then the
response carries `ambiguous: true` with `candidates` — distinct products,
not pack sizes, at most ten, each marked with whether one of the
customer's stores has it in stock, available ones first. `match` is nil
and `stores` empty on that path: no price for a product nobody chose.
Erring towards asking is deliberate — a tap versus the wrong biscuit.

To act on a pick, /lookup now accepts `brand` + `catalogueid` instead of a
label and skips recognition entirely (also serves deep links and re-order).
New: ScanRepository.CatalogueRef, resolving via the brand tables discovered
from information_schema, never a name built from the request.

Also: scratch/cataloguedims now reports every vector column, not just
`embedding` — which is how we learned the catalogue also carries
img_vector(1024), filled on 1885 of 2124 rows. SCAN_TO_ORDER.md records
why that column stays unread for now and what would change it, alongside
why the app is not asked to compute vectors on the phone.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

repositories/scanRepository.go

@@ -94,6 +94,9 @@ type ScanRepository interface {
 	VectorSearch(ctx context.Context, vector []float32, limit int) ([]CatalogueHit, error)
 	TextSearch(ctx context.Context, label string, limit int) ([]CatalogueHit, error)
 	VectorSearchAvailable() bool
+	// CatalogueRef is one product named by its catalogue key, with its other
+	// pack sizes after it. Nothing is recognised or scored.
+	CatalogueRef(ctx context.Context, brand string, id int64) ([]CatalogueHit, error)
 
 	// cache
 	CachedVector(ctx context.Context, model, label string) ([]float32, bool)
@@ -519,6 +522,72 @@ func (r *scanRepository) TextSearch(ctx context.Context, label string, limit int
 	return hits, nil
 }
 
+// tableFor resolves a brand the caller named to a real catalogue table.
+//
+// The lookup is against the tables discovered from information_schema, never
+// a string built from the request: table names cannot be parameterised in
+// SQL, so the discovered map is what keeps this from being an injection
+// point. Both the table suffix ("britannia") and a display name ("24 Mantra"
+// → brand_24_mantra) resolve.
+func (r *scanRepository) tableFor(ctx context.Context, brand string) (string, map[string]bool, error) {
+	tables, err := r.brandTables(ctx)
+	if err != nil {
+		return "", nil, err
+	}
+	for _, candidate := range []string{
+		"brand_" + strings.ToLower(strings.TrimSpace(brand)),
+		"brand_" + normaliseBrandKey(brand),
+	} {
+		if cols, ok := tables[candidate]; ok {
+			return candidate, cols, nil
+		}
+	}
+	return "", nil, ErrUnknownBrand
+}
+
+// CatalogueRef reads one product by (brand, id) and appends its other pack
+// sizes — same variant_key where the catalogue assigned one, same name
+// otherwise, matching how the search groups a family.
+//
+// Distance is 0 on every row: nothing here was ranked, the caller said which
+// product they meant.
+func (r *scanRepository) CatalogueRef(ctx context.Context, brand string, id int64) ([]CatalogueHit, error) {
+	table, cols, err := r.tableFor(ctx, brand)
+	if err != nil {
+		return nil, err
+	}
+	suffix := strings.TrimPrefix(table, "brand_")
+	columns := hitColumns(suffix, cols)
+
+	var self []CatalogueHit
+	err = r.catalogue.WithContext(ctx).Raw(fmt.Sprintf(
+		`SELECT %s, 0::float8 AS distance FROM %s WHERE id = ?`, columns, table), id).Scan(&self).Error
+	if err != nil {
+		return nil, err
+	}
+	if len(self) == 0 {
+		return nil, nil
+	}
+
+	var siblings []CatalogueHit
+	if cols["variant_key"] && strings.TrimSpace(self[0].VariantKey) != "" {
+		err = r.catalogue.WithContext(ctx).Raw(fmt.Sprintf(
+			`SELECT %s, 0::float8 AS distance FROM %s WHERE variant_key = ? AND id <> ? ORDER BY id`,
+			columns, table), self[0].VariantKey, id).Scan(&siblings).Error
+	} else {
+		err = r.catalogue.WithContext(ctx).Raw(fmt.Sprintf(
+			`SELECT %s, 0::float8 AS distance FROM %s WHERE LOWER(product_name) = LOWER(?) AND id <> ? ORDER BY id`,
+			columns, table), self[0].ProductName, id).Scan(&siblings).Error
+	}
+	if err != nil {
+		// The product itself was found; losing its other sizes is the smaller
+		// failure and the caller asked for this one.
+		log.Printf("scan: could not read pack sizes of %s#%d: %v", brand, id, err)
+		return self, nil
+	}
+	return append(self, siblings...), nil
+}
+
 func sortedKeys(m map[string]map[string]bool) []string {
 	keys := make([]string, 0, len(m))
 	for k := range m {