fix(orders): record discount usage against the table the code actually came from

[MOHITKOURAV01]

What this fixes

Closes #1712

A discount code can resolve from either of two independent tables — coupons (ledger: coupon_usage) or promo_codes (ledger: promo_usage). Both use INT AUTO_INCREMENT primary keys starting at 1, so their id spaces overlap completely. The usage-tracking block in createOrderService() treated whatever id it was holding as a promo_codes id:

const appliedPromoId = appliedPromo ? appliedPromo.id : null;   // could be either table
...
if (appliedPromoId) {
    await connection.query("UPDATE promo_codes SET usage_count = usage_count + 1 WHERE id = ?", [appliedPromoId]).catch(() => {});
    if (user_id) {
        await connection.query("INSERT INTO promo_usage (promo_id, ...) VALUES (?, ...)", [appliedPromoId, ...]).catch(() => {});
    }
}

Redeeming coupon id 3 incremented promo id 3's usage_count and filed a promo_usage row for a promo nobody used. Both writes end in .catch(() => {}), so nothing surfaced anywhere.

The paired recordCouponUsage() call above it has the mirror-image problem — it runs UPDATE coupons SET used_count = used_count + 1 for a promo_codes redemption. So a single redemption wrote to three tables, two of them wrong.

couponService.validateCoupon() has always reported the answer — isPromoTable on the returned object — and the order flow was throwing it away at line 466.

What changed

New: backend/services/discountUsageService.js

The single place that decides which ledger a redemption belongs to:

• resolveDiscountSource(promo, declaredSource) — trusts an explicitly declared source first, then isPromoTable, then infers from discount_type / discount_value (columns only promo rows carry). Returns null when there is genuinely nothing to go on.
• recordDiscountUsage(connection, params) — routes to promo_codes + promo_usage, or delegates to couponService.recordCouponUsage() for coupons. Returns the source it wrote to.

backend/services/order.service.js

• Carries appliedPromoSource alongside appliedPromo, set from couponVal.coupon.isPromoTable on the coupon path and to promo_codes on the legacy validatePromo() path.
• The ~20-line tracking block is replaced by one recordDiscountUsage() call.

Deliberate choices

• An unattributable code writes nothing. Guessing is worse than abstaining: writing to the wrong ledger silently burns another campaign's budget and leaves no signal. It logs a warning instead.
• Failures log rather than vanish. The two .catch(() => {}) swallows are replaced with logger.error. Neither still fails the order — the order is already paid for — but a broken ledger write is now visible.
• A guest still bumps the global counter, and still writes no per-account row. promo_usage.user_id needs an identity a guest does not have.
• The routing lives in its own module rather than inline, because createOrderService is long enough that the branch could not be tested without standing up a full order.

Tests

New backend/tests/discountUsageService.test.js, 18 cases:

• source resolution, including the "return null rather than guess" path
• a coupons redemption never touching promo_codes or promo_usage, and vice-versa
• the guest path, both failure paths, and the no-op guards
• three assertions over order.service.js itself, pinning that the direct UPDATE promo_codes / INSERT INTO promo_usage statements are gone and that the source comes from the validator rather than from the id

Tests:       18 passed, 18 total

Ran the surrounding suites (order, checkout, promo, coupon, cart — 474 tests) with no new failures. cartBulkSelection.test.js has one failure that reproduces unchanged on main and is unrelated to this branch.

Scope

backend/services/order.service.js, one new service, one new test file. No schema, route or frontend changes.

---

CI note

Syntax check is red on this branch, and on every open PR in the repo right now. The single failure is inherited from main:

❌ 1 of 653 JavaScript file(s) failed to parse:
  frontend/scripts/shop.js:2499
      Unexpected end of input

No file this PR touches is implicated — shop.js is untouched here. It is tracked as #1696 with a fix open in #1701. Because Syntax check gates the rest of the workflow, Backend tests and Server boots report skipping rather than running; both suites pass locally on this branch.

The red Vercel check is the repo-wide Authorization required to deploy (team bhuvanshs-projects) and is unrelated to this change.

[hydra-maintainer]

🔍 Quality Gate Report

✅ All quality gates passed!

Status	Check	Details
✅	Linked Issue	PR description references a closing issue ✅

[hydra-maintainer]

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🟡 PR Health Score: 55/100

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