feat(backend): guest-facing Wahl overview endpoint + dev seed

GET /api/wahl/guest/overview (guest JWT) returns the open Wahlen for the
guest's KC, each with its workshops and the guest's own current priorities
(null if not yet submitted) — everything the client needs to render the
Wahl form without any LT-only endpoint. Verified end to end against a local
Postgres (guest login -> overview -> submit -> re-fetch).

prisma/seed-dev.js: minimal dev fixture (one KC "DEV123" + Gemeinde + open
Wahl with three workshops).

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

src/wahl/wahl.controller.ts

@@ -69,6 +69,15 @@ export class WahlController {
     return this.wahl.createForceZuteilung(wahlId, dto.teilnehmerId, dto.workshopId);
   }
 
+  /// Everything a guest needs to fill in the Wahl: open Wahlen for their KC,
+  /// each with its workshops and the guest's own current priorities (if any).
+  @Get('guest/overview')
+  @UseGuards(AuthGuard('guest'))
+  guestOverview(@Req() req: GuestAuthenticatedRequest) {
+    const guest = req.user!;
+    return this.wahl.guestOverview(guest.kcId, guest.guestId);
+  }
+
   /// Guests submit their own workshop preferences (guest JWT, not Authentik).
   @Post(':wahlId/teilnehmer')
   @UseGuards(AuthGuard('guest'))