feat: Promo Codes and Coupon System

[swetalin-10]

Closes #138

What's changed

Backend

BACKEND/models/coupon.model.js (new)

Field	Type	Notes
code	String	Unique, uppercase, 3–20 chars
type	'percentage' | 'flat'	Discount type
value	Number	% (1–100) or flat $ amount
minOrderAmount	Number	Minimum cart total to apply
maxUses	Number | null	null = unlimited
usedCount	Number	Auto-incremented on fulfillment
expiresAt	Date | null	null = never expires
isActive	Boolean	Soft-disable without deleting

BACKEND/controllers/coupon.controller.js (new)

• POST /api/coupons/validate — public; checks code validity, expiry, usage limit, and minimum order; returns discount and finalTotal
• POST /api/coupons — admin: create coupon
• GET /api/coupons — admin: list all coupons
• PATCH /api/coupons/:code/deactivate — admin: soft-deactivate

BACKEND/controllers/checkout.controller.js

• Accepts optional couponCode alongside items
• Re-validates the coupon server-side (not trusting frontend discount amount)
• Applies discount by scaling each line item's unit_amount proportionally so Stripe sees the correct discounted total
• Stores couponCode and discountAmount in Stripe session metadata
• Webhook increments usedCount after successful payment fulfillment

---

Frontend — Cart Drawer (Navbar.jsx)

Promo code input — visible only when cart has items:

• Text input auto-uppercases as you type; press Enter or click Apply
• Calls POST /api/coupons/validate with current cart total
• On success: input replaced by a dismissable green tag showing code + savings
• On error: toast with the reason (expired, limit reached, minimum not met, etc.)

Updated totals section:

• Strikethrough original price shown when a coupon is applied
• Discounted total displayed in cyan
• couponCode forwarded to POST /api/checkout on proceed

---

Creating coupons (admin)

# 20% off all orders
POST /api/coupons
{ "code": "SUMMER20", "type": "percentage", "value": 20 }

# $10 flat off orders over $50
POST /api/coupons
{ "code": "SAVE10", "type": "flat", "value": 10, "minOrderAmount": 50 }

# 15% off, max 100 uses, expires end of year
POST /api/coupons
{ "code": "HOLIDAY15", "type": "percentage", "value": 15, "maxUses": 100, "expiresAt": "2026-12-31" }

---

Screenshots

Entering a promo code

After applying — discount reflected in total

[vercel]

@swetalin-10 is attempting to deploy a commit to the niharika-mente's projects Team on Vercel.

A member of the Team first needs to authorize it.

[Aamod-Dev]

@swetalin-10 can u resolve conflicts

[Aamod-Dev]

Looks good, approved!

[Aamod-Dev]

Looks good, approved!