v0.2.0:20 — Multi-policy scope for discount codes

A discount code can now apply to a subset of policies on a product
(e.g. "Patron and Pro but not Creator") instead of being limited to
exactly one policy or the entire product.

- Migration 0018 adds `applies_to_policy_ids_json` (nullable JSON array
  of policy ids). Legacy `applies_to_policy_id` stays as the singular
  fallback when the JSON column is empty/NULL.
- `DiscountCode::allowed_policy_ids()` helper unifies multi + singular
  into one Vec. Purchase + preview scope checks consult it.
- `find_applicable_featured_discount` now narrows multi-policy
  candidates in Rust (small candidate set; index-friendly SQL would
  require json_each, deferred).
- Admin API: `POST /v1/admin/discount-codes` accepts `policy_slugs`
  (array) alongside the existing `policy_slug` (singular). Multi wins
  when both are present. PATCH does not allow scope edits — same rule
  as the singular field (disable + recreate to re-scope).
- UI: pill multi-select replaces the policy dropdown on the create
  form. Edit modal's scope label renders the comma-separated list.

UI + schema both back-compat: existing codes keep working unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

licensing-service/src/api/purchase.rs

@@ -292,13 +292,16 @@ pub async fn start(
                 ));
             }
         }
-        // If the code is restricted to a specific policy and a tier was
-        // selected, they must match. If no tier was selected, the code is
-        // implicitly applied to the product's default policy at issuance
-        // time, which we accept here (v0.1.0:27+).
-        if let Some(restricted_pid) = &code.applies_to_policy_id {
+        // If the code is restricted to one or more policies and a tier
+        // was selected, the chosen tier must be in the allowed set.
+        // `allowed_policy_ids()` unifies the multi-policy column (0018)
+        // and the legacy singular column. If no tier was selected, the
+        // code is implicitly applied to the product's default policy at
+        // issuance time, which we accept here (v0.1.0:27+).
+        let allowed = code.allowed_policy_ids();
+        if !allowed.is_empty() {
             if let Some(chosen) = &chosen_policy {
-                if restricted_pid != &chosen.id {
+                if !allowed.iter().any(|p| *p == chosen.id) {
                     return Err(AppError::BadRequest(
                         "discount code does not apply to the selected tier".into(),
                     ));