niletech.fi: load under 1 second (plan + discussion)

Imported from GitHub issue El-SaMa/oma#202 by @El-SaMa.

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Goal from the operator 2026-10-14: niletech.fi pages load in under 1 s. Today every page is 2-4 s (after the 2026-10-14 English fix; before it, English pages took 18-25 s).

Findings so far:

• No page cache: every response is X-FastCGI-Cache: BYPASS. Polylang sets a pll_language cookie on every page, so nginx never serves cached HTML. This is the biggest win: cached anonymous pages are about 0.1-0.3 s.
• 2,994 product taxonomies (attribute duplicates, see the niletech-attr-dupes notes). Every product query and admin screen pays for them.
• Heavy stack: WoodMart + Elementor + Elementor Pro + Ultimate Elementor + Polylang + many plugins; menus are built per request.
• Needless writes per request (small): _elementor_design_system_sync_css_meta delete+insert, a Fluent license option, and on English pages 3 WoodMart default_term_* options.

Plan to discuss before touching the live shop:

• Page cache for anonymous visitors without a cart: don't let the Polylang cookie break caching (language from URL only), and bypass rules for cart, checkout, my-account, logged-in users and WooCommerce session cookies.
• Cache purge on product, price and stock changes (CataSync pushes, Woo saves).
• Merge or clean up the duplicate attribute taxonomies.
• Stop the per-request option writes.
• Plugin and asset audit (Elementor/WoodMart CSS/JS weight, unused plugins).
• Measure before and after: TTFB + full load for home, category, product, search, cart and checkout in FI and EN, plus a live add-to-cart → checkout test.

Replaces #197.