feat(backend): Wahl-Phasen, Verantwortliche-Invites, Auth fixes

- New VerantwortlicheInvite model: LT-issued invites so a person can
  register as Gemeinde Verantwortliche(r) for a specific Gemeinde,
  skipping the self-registration approval step.
- Wahl/Workshop/Teilnehmer gain phase support (phasenAnzahl,
  beschreibung), mirroring the WP plugin's multi-phase elections.
  Teilnehmer unique constraint now scoped per phase.
- Auth: team login + guest auth adjustments, spec coverage.
- sync.service.ts: register VerantwortlicheInvite as a synced model.
- wahl.service.ts: submitTeilnehmer updated for the new phase-scoped
  unique key.
- client: login/home screen rework, new theme.dart, FCM web tweaks.
- .gitignore: ignore .DS_Store.

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

backend/prisma/migrations/20260911100000_wahl_phasen_and_verantwortliche_invites/migration.sql

@@ -0,0 +1,43 @@
+-- DropIndex
+DROP INDEX "Teilnehmer_wahlId_guestAccountId_key";
+
+-- AlterTable
+ALTER TABLE "Wahl" ADD COLUMN     "beschreibung" TEXT,
+ADD COLUMN     "phasenAnzahl" INTEGER NOT NULL DEFAULT 1;
+
+-- AlterTable
+ALTER TABLE "Workshop" ADD COLUMN     "beschreibung" TEXT,
+ADD COLUMN     "phase" INTEGER NOT NULL DEFAULT 1;
+
+-- AlterTable
+ALTER TABLE "Teilnehmer" ADD COLUMN     "phase" INTEGER NOT NULL DEFAULT 1;
+
+-- CreateTable
+CREATE TABLE "VerantwortlicheInvite" (
+    "id" TEXT NOT NULL,
+    "kcId" TEXT NOT NULL,
+    "gemeindeId" TEXT NOT NULL,
+    "token" TEXT NOT NULL,
+    "email" TEXT,
+    "maxUses" INTEGER,
+    "usedCount" INTEGER NOT NULL DEFAULT 0,
+    "expiresAt" TIMESTAMP(3),
+    "revokedAt" TIMESTAMP(3),
+    "createdByUserId" TEXT NOT NULL,
+    "createdAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "VerantwortlicheInvite_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE UNIQUE INDEX "VerantwortlicheInvite_token_key" ON "VerantwortlicheInvite"("token");
+
+-- CreateIndex
+CREATE UNIQUE INDEX "Teilnehmer_wahlId_guestAccountId_phase_key" ON "Teilnehmer"("wahlId", "guestAccountId", "phase");
+
+-- AddForeignKey
+ALTER TABLE "VerantwortlicheInvite" ADD CONSTRAINT "VerantwortlicheInvite_kcId_fkey" FOREIGN KEY ("kcId") REFERENCES "Kc"("id") ON DELETE CASCADE ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "VerantwortlicheInvite" ADD CONSTRAINT "VerantwortlicheInvite_gemeindeId_fkey" FOREIGN KEY ("gemeindeId") REFERENCES "Gemeinde"("id") ON DELETE CASCADE ON UPDATE CASCADE;
+