fix(api): validate quantity on POST /api/user-cards

Match decks handler parseInt/NaN guard so non-numeric quantities cannot
corrupt user_cards row counts from scanner add paths.

Co-authored-by: Cursor <cursoragent@cursor.com>

.convoys/redesign-scanner-flow/audit-redesign-scanner-flow-44.md

@@ -24,8 +24,8 @@ Reports posted to [PR #44](https://github.com/varutasu/tcg-vault/pull/44#issueco
 
 | Slug | Priority | Source |
 | --- | --- | --- |
-| `scanner-redesign-a11y-fixes` | P1 | **In progress** — PR pending |
+| `scanner-redesign-a11y-fixes` | P1 | **RESOLVED** — PR #45 |
-| `scanner-user-cards-quantity-guard` | P2 | Reviewer — parseInt/NaN guard parity with decks handler |
+| `scanner-user-cards-quantity-guard` | P2 | **In progress** — PR pending |
 | `test-scanner-redesign-surfaces` | P2 | Reviewer — unit tests for new components + upload route |
 | `document-condition-foil-destination-semantics` | P3 | Reviewer — clarify or migrate condition/foil for collection/deck rows |
 | `camera-scanner-token-cleanup` | P3 | Design system — replace hardcoded hex overlay colors in CameraScanner |

.convoys/scanner-user-cards-quantity-guard.md

@@ -0,0 +1,25 @@
+---
+name: scanner-user-cards-quantity-guard
+classification: fix
+success_metric: |
+  POST /api/user-cards rejects non-numeric and sub-1 quantity with 400, matching
+  the decks/[id]/cards handler contract.
+depends_on:
+  - redesign-scanner-flow
+status: open
+created: 2026-05-27
+---
+
+# Convoy: scanner-user-cards-quantity-guard
+
+P2 follow-up from `audit-redesign-scanner-flow-44` reviewer report.
+
+## Scope
+
+- `pages/api/user-cards.js` — `parseInt(quantity, 10)` + NaN / `< 1` guard on POST
+
+## Acceptance criteria
+
+1. `quantity: "abc"` → 400 `Quantity must be at least 1`
+2. `quantity: 0` → 400
+3. Valid integer ≥ 1 uses parsed value for INSERT and UPDATE increment

pages/api/user-cards.js

@@ -21,6 +21,11 @@ export default async function handler(req, res) {
         return res.status(400).json({ error: 'Card ID is required' });
       }
 
+      const parsedQuantity = parseInt(quantity, 10);
+      if (Number.isNaN(parsedQuantity) || parsedQuantity < 1) {
+        return res.status(400).json({ error: 'Quantity must be at least 1' });
+      }
+
       const scanImageUrl =
         typeof scanImageUrlRaw === 'string' && scanImageUrlRaw.trim().length > 0
           ? scanImageUrlRaw.trim()
@@ -34,7 +39,7 @@ export default async function handler(req, res) {
 
       if (existingResult.rows.length > 0) {
         // Update quantity; preserve existing scan image unless a new URL is supplied
-        const newQuantity = existingResult.rows[0].quantity + quantity;
+        const newQuantity = existingResult.rows[0].quantity + parsedQuantity;
         await sql`
           UPDATE user_cards 
           SET quantity = ${newQuantity},
@@ -46,7 +51,7 @@ export default async function handler(req, res) {
         // Insert new record
         await sql`
           INSERT INTO user_cards (user_id, card_id, quantity, condition, is_foil, scan_image_url)
-          VALUES (${user.userId}, ${cardId}, ${quantity}, ${condition}, ${is_foil}, ${scanImageUrl})
+          VALUES (${user.userId}, ${cardId}, ${parsedQuantity}, ${condition}, ${is_foil}, ${scanImageUrl})
         `;
       }