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>

Dockerfile

@@ -8,6 +8,11 @@
 # --- 1. Backend build ---------------------------------------------------------
 FROM node:20-bookworm-slim AS api-build
 WORKDIR /src
+# Prisma detects the OpenSSL version at `generate` time to pick the matching
+# query engine binary; without OpenSSL present here it silently defaults to
+# openssl-1.1.x, which then fails to load in the runtime stage (openssl 3.0.x).
+RUN apt-get update && apt-get install -y --no-install-recommends openssl \
+  && rm -rf /var/lib/apt/lists/*
 COPY backend/package.json backend/package-lock.json ./
 RUN npm ci
 COPY backend/ ./