Major refactor: auth, bug fixes, security hardening, modular routes

Bug fixes
- Sync database.js schema with actual production DB (tarifoptionen
  shape was wrong; new installs would have broken).
- Fix SQL syntax error `SELECT to.*` (reserved keyword) in single-
  schaltung provision endpoint, plus the dead fallback hierarchy
  that selected columns that were never in the query.
- Fix PUT /api/tarifoptionen/:id writing to non-existent column
  provision_betrag (should be provision_euro).
- Fix race condition in PUT /api/schaltungen/:id where forEach with
  async db.run inside a transaction could send duplicate responses.
- Delete dokumente rows explicitly before deleting schaltung
  (existing DBs lack ON DELETE CASCADE on dokumente FK).
- Replace hardcoded basisBetrag=100 for percent rules; basis is now
  configurable per rule via the new basis_euro column.

Security
- Multi-user authentication: users table, bcryptjs, express-session
  with SQLite store, login/logout/me/setup/change-password endpoints.
- Auth gate on all /api/* except /api/auth/*.
- CLI to create users: npm run create-user -- <user> <pass> [--admin].
- helmet for security headers.
- CORS restricted to ALLOWED_ORIGINS env (default same-origin).
- Rate limits: 300/min global, 20/15min on login.
- Path-traversal validation on document download/delete.
- XSS hardening in frontend: escapeHtml helper applied to all
  DB-derived innerHTML interpolations; inline onclick handlers that
  embedded user-controlled strings replaced with data-* attrs +
  addEventListener.

Architecture
- Provision calculation centralised in provisionService.js as the
  single source of truth (was duplicated and inconsistent).
- server.js cut from 2138 to ~140 lines; routes split into per-
  resource modules under routes/.
- Promise wrapper db.js for sqlite3 callbacks; async/await in routes.
- middleware/ for auth and async error handling.

Tests
- 14 unit tests for provisionService covering fallback hierarchy,
  percent calculation, tarifoptionen, hardware logic, employee
  commission. All green.

Config
- dotenv loads .env.local at startup (gitignored).
- .gitignore extended for sessions.db and .env*.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

middleware/asyncHandler.js

@@ -0,0 +1,5 @@
+// Wraps an async route handler so thrown errors flow to Express's error middleware
+// instead of becoming unhandled rejections.
+module.exports = function asyncHandler(fn) {
+    return (req, res, next) => Promise.resolve(fn(req, res, next)).catch(next);
+};

middleware/auth.js

@@ -0,0 +1,17 @@
+// Auth middleware: requireLogin gates API routes, requireAdmin restricts admin actions.
+
+function requireLogin(req, res, next) {
+    if (req.session && req.session.userId) {
+        return next();
+    }
+    return res.status(401).json({ error: 'Nicht angemeldet' });
+}
+
+function requireAdmin(req, res, next) {
+    if (req.session && req.session.userId && req.session.isAdmin) {
+        return next();
+    }
+    return res.status(403).json({ error: 'Admin-Rechte erforderlich' });
+}
+
+module.exports = { requireLogin, requireAdmin };