fix: add billing types and fix Supabase type casting for admin routes

- Add src/types/billing.ts with Payment, Coupon, CreditTransaction, Invoice types
- Cast all Supabase query results through 'unknown' for untyped billing tables
- All routes now build cleanly with strict TypeScript checking

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

frontend/src/app/api/admin/credits/route.ts

@@ -1,6 +1,7 @@
 import { NextResponse } from "next/server";
 import { getSupabaseAdmin } from "@/lib/admin";
 import { requireAdmin } from "@/lib/apiAuth";
+import type { CreditTransaction } from "@/types/billing";
 
 export async function GET(request: Request) {
   const authResult = await requireAdmin(request);
@@ -37,7 +38,7 @@ export async function GET(request: Request) {
 
       return NextResponse.json({
         organization: orgResult.data,
-        transactions: transactionsResult.data || [],
+        transactions: (transactionsResult.data || []) as unknown as CreditTransaction[],
         pagination: {
           page,
           limit,
@@ -111,7 +112,7 @@ export async function POST(request: Request) {
     }
 
     // Insert transaction and update balance atomically
-    const { data: transaction, error: txError } = await supabase
+    const { data, error: txError } = await supabase
       .from("credit_transactions")
       .insert({
         organization_id,
@@ -126,6 +127,7 @@ export async function POST(request: Request) {
       .single();
 
     if (txError) throw txError;
+    const transaction = data as unknown as CreditTransaction;
 
     const { error: updateError } = await supabase
       .from("organizations")