fix(coupons): enforce start_date, deleted_at and per_user_limit in coupon validation

[MOHITKOURAV01]

What this fixes

Closes #1711

validateCoupon() looked at is_active, an end date, the global usage_limit and minimum_order_amount. Three columns the coupons table has carried since migrations/0001_baseline_schema.sql were never read at all:

Column	Consequence of skipping it
start_date TIMESTAMP NOT NULL	A campaign scheduled to open next month discounts orders today.
deleted_at DATETIME	A soft-deleted coupon keeps validating and keeps discounting.
per_user_limit INT DEFAULT 1	A "one per customer" coupon is unlimited per customer.

The userId argument was accepted and documented as "ID of the calling user" and then never referenced anywhere in the function body — which is exactly why the per-account cap was unenforceable.

What changed

backend/services/couponService.js

• Start date. Rejects with "Coupon code is not active yet" when start_date / starts_at / valid_from is still in the future. All three spellings are accepted because the coupons and promo_codes tables name the column differently.
• Soft deletion. A non-null deleted_at is rejected as an invalid code. Deliberately reuses the "Invalid coupon code" wording rather than announcing that a coupon used to exist.
• Per-account limit. When a userId is supplied, prior redemptions are counted and checked against per_user_limit (or usage_limit_per_user on a promo row). Counting reads coupon_usage for a coupons-table code and promo_usage for a promo_codes code, so the two ledgers stay separate.
• The ledger is now written. coupon_usage has existed since the baseline schema and nothing in the codebase ever inserted into it — without a row there is nothing for the cap to count. recordCouponUsage() now files one. Its new options argument is optional, so the existing three-argument call in order.service.js keeps working with no change.

Deliberate choices

• Unparseable dates do not reject. parseDate() returns null for junk rather than a date, so a malformed start_date cannot make a live coupon look "not yet active". Rejecting a valid coupon is the worse failure here.
• A zero or negative per_user_limit means uncapped, not "nobody may use this". A mis-seeded row should not lock out every shopper.
• A missing ledger table counts as zero redemptions. If coupon_usage has not been migrated yet, checkout still completes and the global usage_limit remains the backstop — the alternative is a hard checkout failure on a schema gap.
• Guests skip the per-account check entirely. coupon_usage.user_id is NOT NULL and a guest has no stable identity to cap against, so no query is issued and no row is written.
• A failed ledger write never fails a paid order — it is logged and swallowed.

Tests

New backend/tests/couponLifecycle.test.js, 21 cases across start-date handling, soft deletion, per-account limits (including the coupons-vs-promo table split and the guest path), ledger writes, backwards compatibility of the three-argument call, and date parsing. They run against a stub connection, so no MySQL is needed in CI.

Verified the guards are meaningful: with the service reverted and only the tests applied, 11 of the 21 fail. With the fix in place all 21 pass.

Tests:       21 passed, 21 total

Scope

backend/services/couponService.js and one new test file. No route, schema or frontend changes — the call sites keep their current signatures.

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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.

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🔍 Quality Gate Report

✅ All quality gates passed!

Status	Check	Details
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💡 Suggested reviewers based on relevant file history: @Aditya8369, @Pcmhacker-hero

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🟡 PR Health Score: 55/100

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