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>

backend/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'))

backend/src/wahl/wahl.service.ts

@@ -22,6 +22,40 @@ export class WahlService {
     return this.prisma.wahl.findMany({ where: { kcId } });
   }
 
+  /// Guest-facing view: open Wahlen for the guest's KC, each with its
+  /// workshops and the guest's own current priorities (null if not submitted).
+  async guestOverview(kcId: string, guestAccountId: string) {
+    const [kc, wahlen] = await Promise.all([
+      this.prisma.kc.findUnique({ where: { id: kcId }, select: { id: true, name: true } }),
+      this.prisma.wahl.findMany({
+        where: { kcId, isOpen: true },
+        orderBy: { createdAt: 'asc' },
+        include: {
+          workshops: {
+            select: { id: true, name: true, kapazitaet: true },
+            orderBy: { name: 'asc' },
+          },
+          teilnehmer: {
+            where: { guestAccountId },
+            select: { prioritaeten: true },
+          },
+        },
+      }),
+    ]);
+
+    return {
+      kc,
+      wahlen: wahlen.map((w) => ({
+        id: w.id,
+        name: w.name,
+        datumsSchluessel: w.datumsSchluessel,
+        teil: w.teil,
+        workshops: w.workshops,
+        meinePrioritaeten: (w.teilnehmer[0]?.prioritaeten as string[] | undefined) ?? null,
+      })),
+    };
+  }
+
   async createWorkshop(
     wahlId: string,
     name: string,