feat: Products module — SKU CRUD, search, Firestore repository

[cto-new]

Summary

Implements the Products module — the core KPI for StockFlow (Active SKUs tracked).

Changes

New Files

• product.controller.ts — 6 endpoints: GET /products, GET /products/:id, GET /products/:sku, POST /products, PATCH /products/:id, DELETE /products/:id
• product.repository.ts — FirestoreProductRepository implementing IProductRepository with paginated queries, search, category filtering, sort, SKU uniqueness
• create-product.dto.ts — Validation DTO with class-validator
• update-product.dto.ts — Validation DTO with optional fields
• query-product.dto.ts — Query DTO with pagination, search, filter, sort params
• product.service.spec.ts — 11 unit tests

Modified Files

• product.service.ts — Proper NotFoundException/ConflictException, SKU conflict check
• inventory.module.ts — Registers ProductController and FirestoreProductRepository

Key Architecture Decisions

• Firestore collection products, queries via composite indexes
• nameLower field stored for case-insensitive prefix search
• Soft delete via isActive flag
• Pagination via Firestore offset+limit mapped to page/limit
• Auth guards from AuthModule applied to all product endpoints

Testing

• All 37 tests pass (16 auth + 11 products + 6 roles + 4 pagination)
• TypeScript compiles cleanly