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/invoices/route.ts

@@ -3,6 +3,7 @@ import { getSupabaseAdmin } from "@/lib/admin";
 import { requireAdmin } from "@/lib/apiAuth";
 import { sendEmail } from "@/lib/email";
 import { invoiceEmail } from "@/lib/email-templates";
+import type { Invoice } from "@/types/billing";
 
 export async function GET(request: Request) {
   const authResult = await requireAdmin(request);
@@ -29,11 +30,13 @@ export async function GET(request: Request) {
     if (orgId) query = query.eq("organization_id", orgId);
     if (status) query = query.eq("status", status);
 
-    const { data: invoices, count, error } = await query;
+    const { data, count, error } = await query;
     if (error) throw error;
 
+    const invoices = (data || []) as unknown as Invoice[];
+
     return NextResponse.json({
-      invoices: invoices || [],
+      invoices,
       pagination: { page, limit, total: count || 0, pages: Math.ceil((count || 0) / limit) },
     });
   } catch (error) {
@@ -68,7 +71,7 @@ export async function POST(request: Request) {
       .like("invoice_number", `${prefix}%`);
     const seq = String((count || 0) + 1).padStart(4, "0");
 
-    const { data: invoice, error } = await supabase
+    const { data, error } = await supabase
       .from("invoices")
       .insert({
         invoice_number: `${prefix}-${seq}`,
@@ -85,6 +88,7 @@ export async function POST(request: Request) {
       .single();
 
     if (error) throw error;
+    const invoice = data as unknown as Invoice;
 
     return NextResponse.json({ invoice });
   } catch (error) {
@@ -109,17 +113,19 @@ export async function PATCH(request: Request) {
 
     // Send invoice via email
     if (action === "send") {
-      const { data: invoice } = await supabase
+      const { data } = await supabase
         .from("invoices")
         .select("*, organization:organizations(id, name, billing_email)")
         .eq("id", id)
         .single();
 
+      const invoice = data as unknown as Invoice | null;
+
       if (!invoice) {
         return NextResponse.json({ error: "Invoice not found" }, { status: 404 });
       }
 
-      const org = invoice.organization as { id: string; name: string; billing_email?: string } | null;
+      const org = invoice.organization;
       const recipientEmail = org?.billing_email;
 
       if (!recipientEmail) {
@@ -131,12 +137,12 @@ export async function PATCH(request: Request) {
 
       const appUrl = process.env.NEXT_PUBLIC_APP_URL || "http://localhost:3000";
       const template = invoiceEmail({
-        organizationName: org.name,
+        organizationName: org?.name || "Unknown",
         invoiceNumber: invoice.invoice_number,
         amount: invoice.amount,
         currency: invoice.currency,
         dueDate: invoice.due_date,
-        items: (invoice.items as Array<{ description: string; amount: number }>) || [],
+        items: (invoice.items) || [],
         dashboardUrl: `${appUrl}/dashboard/settings`,
       });
 
@@ -161,7 +167,7 @@ export async function PATCH(request: Request) {
 
     // Mark as paid
     if (action === "mark_paid") {
-      const { data: invoice, error } = await supabase
+      const { data, error } = await supabase
         .from("invoices")
         .update({ status: "paid", paid_at: new Date().toISOString(), updated_at: new Date().toISOString() })
         .eq("id", id)
@@ -169,6 +175,7 @@ export async function PATCH(request: Request) {
         .single();
 
       if (error) throw error;
+      const invoice = data as unknown as Invoice;
       return NextResponse.json({ invoice });
     }
 
@@ -179,7 +186,7 @@ export async function PATCH(request: Request) {
       if (key in updates) safeUpdates[key] = updates[key];
     }
 
-    const { data: invoice, error } = await supabase
+    const { data, error } = await supabase
       .from("invoices")
       .update(safeUpdates)
       .eq("id", id)
@@ -187,6 +194,7 @@ export async function PATCH(request: Request) {
       .single();
 
     if (error) throw error;
+    const invoice = data as unknown as Invoice;
 
     return NextResponse.json({ invoice });
   } catch (error) {