Replace the single is_free_member flag with a real member-tier system

Design doc §07 extension: admin-configurable tiers, each with an
arbitrary JSON benefit set, instead of one hardcoded boolean. Ships two
benefit types on top of the new Membership\Tier lookup: the existing
free-shipping threshold (refactored, not just left on the old flag) and
a new "$10 off orders $100+, members only" cart discount — bundling
purchases into one $100+ order saves BooksRun's flat per-order shipping
fee outright, more than covering the discount.

Researched official/well-supported WooCommerce plugins first (Memberships,
Points and Rewards, Smart Coupons) — none combine tiers with configurable
per-tier benefit sets, so building custom on the existing wp_bsc_member
table was the right call.

Registration is enabled via WooCommerce's own native My Account signup
(deploy.sh) rather than a bespoke form, with a marketing-consent checkbox
added via the standard extension hooks. Admin management: new `wp
bookstore tier-*` CLI commands plus a WooCommerce Settings screen (this
plugin's first admin UI). Migration drops and recreates bsc_member rather
than backfilling — pre-launch, only synthetic data exists anywhere.

Live-verified beyond the unit suite: real migration run, CLI commands
end-to-end, a real registration submitted through the actual WC form
(with and without consent), and the settings screen edited/created/
deleted tiers through real authenticated form POSTs with real nonces.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MqhgnCRjjVLFnZ3Jpn93Vg

deploy/deploy.sh

@@ -129,6 +129,12 @@ echo "==> enabling WooCommerce HPOS (custom order tables)"
 $WP option update woocommerce_custom_orders_table_enabled yes
 $WP option update woocommerce_custom_orders_table_data_sync_enabled yes
 
+# Membership\Registration hooks onto WooCommerce's own My Account
+# registration form (design doc §07) rather than a bespoke signup form —
+# it's disabled by default, so nothing shows up without this.
+echo "==> enabling WooCommerce account registration (member signup)"
+$WP option update woocommerce_enable_myaccount_registration yes
+
 # WooCommerce's default (unset -> '1:1') force-crops every product
 # thumbnail to a square — confirmed directly: a 335x500 portrait book cover
 # was landing at a hard-cropped 300x300 shop-grid thumbnail, cutting off