V102: Catalogue discount + smart catalogues.

Two related catalogue features shipped together:

Discount

Mirrors the markup columns added in V89 (catalogue) and V97 (item override):

• phoenix_kit_cat_catalogues.discount_percentage DECIMAL(7, 2) NOT NULL DEFAULT 0 — the catalogue-wide default discount applied on top of the post-markup sale price.
• phoenix_kit_cat_items.discount_percentage DECIMAL(7, 2) — nullable per-item override. NULL = inherit the catalogue's discount; any set value (including 0) overrides the catalogue's discount for that item.

The pricing chain becomes base → markup → discount:

sale_price   = base_price * (1 + effective_markup   / 100)
final_price  = sale_price  * (1 -  effective_discount / 100)

Smart catalogues

A smart catalogue's items reference other catalogues with a value + unit (e.g. a "Delivery" item says "5% of Kitchen, 3% of Plumbing, plus $20 flat of Hardware"). Consumers do the math; this module stores the user's intent.

• phoenix_kit_cat_catalogues.kind VARCHAR(20) NOT NULL DEFAULT 'standard' — one of 'standard' (existing behavior) or 'smart' (items reference other catalogues).
• phoenix_kit_cat_items.default_value DECIMAL(12, 4) and default_unit VARCHAR(20) (both nullable) — per-item fallback that applies when a rule row has NULL value/unit.
• New table phoenix_kit_cat_item_catalogue_rules storing one row per (item, referenced_catalogue) pair, with nullable value + unit (inherit from item defaults) and a position INT for UI ordering. Unit vocabulary is open-ended VARCHAR; v1 uses 'percent' and 'flat'. Self- and smart-to-smart references are intentionally allowed — consumers handle cycles at math time.

All operations are idempotent.