feat(ai-credits): real coupon validation endpoint; restore invoice FK integrity

- Add POST /api/ai-credits/coupons/validate: a dry-run of the same
  validate_coupon() check create_order applies, so the checkout UI can
  show a real discount/error before Pay instead of only finding out at
  order-creation time. Never inserts anything.
- Add migration 20260814050000: restores integrity on invoices.payment_id
  after 20260814030000 dropped the hard FK to support polymorphic
  invoice_type (TRACECOIN_PURCHASE -> payments, AI_CREDIT_PURCHASE ->
  ai_credit_orders). A BEFORE INSERT/UPDATE trigger now validates
  payment_id against the right table per invoice_type instead of leaving
  it fully unchecked. Verified against a live Postgres in a scratch
  schema (valid/invalid payment_id, valid/bogus invoice_type).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/payments/src/ai_credits.rs

@@ -41,6 +41,7 @@ pub fn router() -> Router<AppState> {
         .route("/", get(list_packages))
         .route("/order", post(create_order))
         .route("/verify", post(verify_order))
+        .route("/coupons/validate", post(validate_coupon_endpoint))
 }
 
 /// Admin CRUD for the AI credit package catalog -- mounted at
@@ -221,6 +222,87 @@ async fn validate_coupon(
     Ok(Some((discount_paise, coupon_id.to_string(), discounted_price)))
 }
 
+#[derive(Debug, Deserialize)]
+struct ValidateCouponRequest {
+    package_id: String,
+    coupon_code: String,
+}
+
+#[derive(Debug, Serialize)]
+struct ValidateCouponResponse {
+    valid: bool,
+    discount_paise: i32,
+    original_price_inr: i32,
+    final_price_inr: i32,
+    message: Option<String>,
+}
+
+/// Dry-run of the same coupon check create_order applies, so the checkout
+/// UI can show a real discount/error before the user hits Pay instead of
+/// only finding out at order-creation time. Never inserts anything -
+/// redemption is still only recorded when verify_order confirms payment.
+async fn validate_coupon_endpoint(
+    auth: AuthUser,
+    State(state): State<AppState>,
+    Json(payload): Json<ValidateCouponRequest>,
+) -> Json<ValidateCouponResponse> {
+    let package_id = match Uuid::parse_str(&payload.package_id) {
+        Ok(id) => id,
+        Err(_) => {
+            return Json(ValidateCouponResponse {
+                valid: false,
+                discount_paise: 0,
+                original_price_inr: 0,
+                final_price_inr: 0,
+                message: Some("Invalid package id".to_string()),
+            })
+        }
+    };
+
+    let package = sqlx::query_as::<_, AiCreditPackagePriceRow>(
+        "SELECT name, credits, price_inr, applicable_roles FROM ai_credit_packages WHERE id = $1 AND is_active = TRUE",
+    )
+    .bind(package_id)
+    .fetch_optional(&state.pool)
+    .await
+    .ok()
+    .flatten();
+
+    let Some(package) = package else {
+        return Json(ValidateCouponResponse {
+            valid: false,
+            discount_paise: 0,
+            original_price_inr: 0,
+            final_price_inr: 0,
+            message: Some("Invalid or inactive AI credit package".to_string()),
+        });
+    };
+
+    match validate_coupon(&state.pool, auth.user_id, &payload.coupon_code, package_id, package.price_inr).await {
+        Ok(Some((discount_paise, _coupon_id, discounted_price))) => Json(ValidateCouponResponse {
+            valid: true,
+            discount_paise,
+            original_price_inr: package.price_inr,
+            final_price_inr: discounted_price,
+            message: None,
+        }),
+        Ok(None) => Json(ValidateCouponResponse {
+            valid: false,
+            discount_paise: 0,
+            original_price_inr: package.price_inr,
+            final_price_inr: package.price_inr,
+            message: Some("Invalid coupon code".to_string()),
+        }),
+        Err(e) => Json(ValidateCouponResponse {
+            valid: false,
+            discount_paise: 0,
+            original_price_inr: package.price_inr,
+            final_price_inr: package.price_inr,
+            message: Some(e),
+        }),
+    }
+}
+
 async fn create_order(
     auth: AuthUser,
     State(state): State<AppState>,

crates/db/migrations/20260814050000_invoices_payment_id_polymorphic_check.down.sql

@@ -0,0 +1,6 @@
+BEGIN;
+
+DROP TRIGGER IF EXISTS invoices_payment_id_check ON invoices;
+DROP FUNCTION IF EXISTS check_invoices_payment_id();
+
+COMMIT;

crates/db/migrations/20260814050000_invoices_payment_id_polymorphic_check.up.sql

@@ -0,0 +1,32 @@
+-- 20260814030000 dropped invoices_payment_id_fkey to allow AI_CREDIT_PURCHASE
+-- invoices (payment_id -> ai_credit_orders(id)) alongside the pre-existing
+-- TRACECOIN_PURCHASE invoices (payment_id -> payments(id)), but left no
+-- replacement integrity check at all -- any invoice_type could reference a
+-- nonexistent payment_id. Postgres has no native polymorphic FK, so enforce
+-- this with a trigger that picks the right table based on invoice_type.
+
+BEGIN;
+
+CREATE OR REPLACE FUNCTION check_invoices_payment_id() RETURNS TRIGGER AS $$
+BEGIN
+    IF NEW.invoice_type = 'TRACECOIN_PURCHASE' THEN
+        IF NOT EXISTS (SELECT 1 FROM payments WHERE id = NEW.payment_id) THEN
+            RAISE EXCEPTION 'invoices.payment_id % does not reference an existing payments row (invoice_type=TRACECOIN_PURCHASE)', NEW.payment_id;
+        END IF;
+    ELSIF NEW.invoice_type = 'AI_CREDIT_PURCHASE' THEN
+        IF NOT EXISTS (SELECT 1 FROM ai_credit_orders WHERE id = NEW.payment_id) THEN
+            RAISE EXCEPTION 'invoices.payment_id % does not reference an existing ai_credit_orders row (invoice_type=AI_CREDIT_PURCHASE)', NEW.payment_id;
+        END IF;
+    ELSE
+        RAISE EXCEPTION 'invoices.invoice_type % is not a recognized value (expected TRACECOIN_PURCHASE or AI_CREDIT_PURCHASE)', NEW.invoice_type;
+    END IF;
+    RETURN NEW;
+END;
+$$ LANGUAGE plpgsql;
+
+CREATE TRIGGER invoices_payment_id_check
+    BEFORE INSERT OR UPDATE OF payment_id, invoice_type ON invoices
+    FOR EACH ROW
+    EXECUTE FUNCTION check_invoices_payment_id();
+
+COMMIT;