chore: Remove unused billing-related collections, types, and utility modules

- Drop `customers` collection and associated types (`types/index.ts`, `payload.ts`)
- Remove generated `payload-types.ts` file
- Clean up unused exports and dependencies across modules
- Streamline codebase by eliminating redundant billing logic

src/plugin/config.ts

@@ -0,0 +1,76 @@
+
+export const defaults = {
+  paymentsCollection: 'payments'
+}
+
+// Provider configurations
+export interface StripeConfig {
+  apiVersion?: string
+  publishableKey: string
+  secretKey: string
+  webhookEndpointSecret: string
+}
+
+export interface MollieConfig {
+  apiKey: string
+  testMode?: boolean
+  webhookUrl: string
+}
+
+export interface TestProviderConfig {
+  autoComplete?: boolean
+  defaultDelay?: number
+  enabled: boolean
+  failureRate?: number
+  simulateFailures?: boolean
+}
+
+// Customer info extractor callback type
+export interface CustomerInfoExtractor {
+  (customer: any): {
+    name: string
+    email: string
+    phone?: string
+    company?: string
+    taxId?: string
+    billingAddress?: {
+      line1: string
+      line2?: string
+      city: string
+      state?: string
+      postalCode: string
+      country: string
+    }
+  }
+}
+
+// Plugin configuration
+export interface BillingPluginConfig {
+  admin?: {
+    customComponents?: boolean
+    dashboard?: boolean
+  }
+  collections?: {
+    customerRelation?: boolean | string // false to disable, string for custom collection slug
+    customers?: string
+    invoices?: string
+    payments?: string
+    refunds?: string
+  }
+  customerInfoExtractor?: CustomerInfoExtractor // Callback to extract customer info from relationship
+  disabled?: boolean
+  providers?: {
+    mollie?: MollieConfig
+    stripe?: StripeConfig
+    test?: TestProviderConfig
+  }
+  webhooks?: {
+    basePath?: string
+    cors?: boolean
+  }
+}
+
+// Plugin type
+export interface BillingPluginOptions extends BillingPluginConfig {
+  disabled?: boolean
+}