feat: implement authentication flow

Backend:
- AppConfig, AppError, AppState modules for shared infrastructure
- JWT creation/verification with HS256 (jsonwebtoken crate)
- Session management: SHA-256 token hashing, DB-backed sessions
- Auth middleware: AuthUser, RequireHost, RequireAdmin extractors
- POST /api/v1/join: name-only registration, 4-digit PIN + bcrypt hash
- POST /api/v1/recover: PIN-based recovery with 3-attempt lockout (15 min)
- POST /api/v1/admin/login: bcrypt password verification
- DELETE /api/v1/session: logout (session invalidation)
- Migration 006: user PIN lockout columns (failed_pin_attempts, pin_locked_until)
- Models: Event, User (with role enum), Session with all CRUD methods

Frontend:
- api.ts: typed fetch wrapper with automatic Bearer token injection
- auth.ts: JWT/PIN localStorage management with Svelte store
- /join: name entry form with PIN display modal and copy button
- /recover: name + PIN recovery form with saved PIN pre-fill
- /feed: placeholder gallery page with logout
- Root layout: auth initialization on mount
- Root page: redirect to /join or /feed based on auth state

All responses use German language strings as specified.

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

frontend/src/routes/+page.svelte

@@ -1,2 +1,14 @@
-<h1>Welcome to SvelteKit</h1>
-<p>Visit <a href="https://svelte.dev/docs/kit">svelte.dev/docs/kit</a> to read the documentation</p>
+<script lang="ts">
+	import { goto } from '$app/navigation';
+	import { getToken } from '$lib/auth';
+	import { browser } from '$app/environment';
+	import { onMount } from 'svelte';
+
+	onMount(() => {
+		if (getToken()) {
+			goto('/feed');
+		} else {
+			goto('/join');
+		}
+	});
+</script>