**Title:** perf: optimize database operations and backend performance

[saidai-bhuvanesh]

Summary:
This PR addresses performance bottlenecks caused by missing database indexes and redundant query calls across the admin dashboard and checkout flow.

Changes:

• models/Order.js — Added 5 indexes: created_at sort index, compound phone+total+created_at for duplicate detection, status, payment_status, and phone for admin filters
• models/Otp.js — Added compound phone+created_at index for fast OTP lookups
• models/Product.js — Added compound type+id_ref index for instant per-item price resolution during checkout
• api/controllers/orderController.js — Refactored getStats() from 4 separate DB queries to a single $facet aggregation pipeline (4x fewer round-trips)

Tests:

• Added test asserting Order.aggregate is called exactly once when fetching stats
• All 7 integration tests pass with no regressions

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