feat: add flexible billing frequency support for invoices

Assessment groups can now define billing frequency (monthly, quarterly,
annual) with configurable due months and due day. Invoice generation
respects each group's schedule - only generating invoices when the
selected month is a billing month for that group. Adds a generation
preview showing which groups will be billed, period tracking on
invoices, and billing period context in the payments UI.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/modules/invoices/invoices.controller.ts

@@ -16,6 +16,11 @@ export class InvoicesController {
   @Get(':id')
   findOne(@Param('id') id: string) { return this.invoicesService.findOne(id); }
 
+  @Post('generate-preview')
+  generatePreview(@Body() dto: { month: number; year: number }) {
+    return this.invoicesService.generatePreview(dto);
+  }
+
   @Post('generate-bulk')
   generateBulk(@Body() dto: { month: number; year: number }, @Request() req: any) {
     return this.invoicesService.generateBulk(dto, req.user.sub);