Replaces the broken fieldValidation helper (called next() without return,
so execution continued on invalid input) with express-validator chains
applied at the route level. Validation covers: email format, username
constraints, password min length, field lengths, URL and phone formats.
products and tags controllers retain their local fieldValidation pending
a separate ecommerce validation pass.
Helmet adds HTTP security headers (HSTS, X-Frame-Options,
X-Content-Type-Options, etc.). CSP is disabled pending deployment-
specific tuning. Rate limiting (10 req / 15 min per IP) is applied
to POST /login and POST / (registration) to mitigate brute force
and credential stuffing attacks.