Add percentage and fixed amount discount code types

[dparker1005]

What this does

Extends discount codes with formula-based discounts alongside the existing per-level price overrides. Each code now has a discount_type:

• set_price — existing behavior, unchanged; remains the default. Existing codes need no migration.
• percentage — e.g. 20% off the level's regular pricing.
• fixed — e.g. $10 off the level's regular pricing.

Formula codes store one rule on pmpro_discount_codes (discount_value, apply_to_initial, apply_to_recurring) shared across all eligible levels, and inherit everything else — billing cycle, billing limit, trial, expiration — from the level. Sites that need custom structure plus a discount keep using set_price.

How pricing is resolved

New pmpro_get_discounted_level_for_code( $level, $code ) in includes/discount-codes.php is the single source of truth: it confirms the level is eligible via pmpro_discount_codes_levels, applies the rule to the level's current pricing, rounds with pmpro_round_price(), and clamps at $0. All four runtime paths that previously read pmpro_discount_codes_levels as final pricing now use it:

• pmpro_getLevelAtCheckout()
• the AJAX applydiscountcode service (resolved per level before the multi-level combined sum)
• MemberOrder::getMembershipLevel() (the order fallback used by IPN handlers)
• the 2Checkout INS handler

The pmpro_discount_code_level filter still runs at its existing call sites after calculation, so documented filter-based extensions keep receiving final prices. A new pmpro_get_discounted_level_for_code filter runs inside the resolver.

Backwards compatibility for direct table readers

pmpro_discount_codes_levels still holds one row per eligible level for every code type. For formula codes, its pricing columns hold a snapshot of the calculated prices — refreshed on code save and on level save (pmpro_save_membership_level) — so add-ons reading the table directly see plausible, usually-exact prices instead of zeros. Checkout never trusts the snapshot; it always recalculates. This also means the existing gateway-compatibility checks evaluate correct cycle/trial values for formula codes with no changes.

Admin / API

• Edit screen: Discount Type, Discount Amount, and Applies To (initial/recurring) fields; for formula codes the per-level pricing tables collapse to an explanatory note. New Select All / Deselect All buttons for the levels list.
• Codes list table shows the rule (e.g. "20% off initial payment, recurring payments") under the code.
• PMPro_Discount_Code and POST /pmpro/v1/discount_code accept the new fields.
• Schema via dbDelta, db version bumped to 3.9.

Also included

• Fixed pmpro_get_discount_code() to match code strings before IDs so fully numeric codes (e.g. a code literally named 2025) resolve correctly at checkout.
• Replaced duplicate raw code-lookup queries in pmpro_complete_checkout(), pmpro_getLevelAtCheckout(), and the AJAX service with the helper; the resolver now receives the already-fetched code row instead of refetching (once per level in multi-level checkout).

Testing done (local site, Stripe-less flows)

• Percentage and fixed math on one-time, recurring, and trial-bearing levels; fixed discount clamping ($10 off a $1 level → $0); ineligible levels return null; legacy set_price codes resolve identically (including the level_id property MemberOrder depends on).
• Live checkout page renders discounted price text; AJAX apply-code works single-level and sums correctly for multi-level (2+3 → $88 for 20% off $10 + $100).
• Admin form round-trips all three types, JS toggles verified, snapshots written correctly and refreshed after a level price change.
• Numeric code 2025 applies its set_price override at checkout after the lookup-order fix.

Notes for reviewers

• The merge-subscription-delays-set-expiration-dates branch touches the same per-level UI region of adminpages/discountcodes.php; whichever lands second has a small mechanical conflict.
• Pre-existing (untouched here): pmpro_check_discount_code_level_for_gateway_compatibility() queries pmpro_discount_codes_levels by an id column that doesn't exist, so its check-all branch silently passes — worth a separate issue.
• @since TBD placeholders to be filled at release.

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