feat: add teacher system with approval flow

- Teacher registration requires subject selection; account starts pending
- Admin approves/rejects via existing admin panel
- Teacher panel (Materialien, Ankündigungen, Prüfungen, Noten) visible only to approved teachers
- Students see class materials and announcements via sidebar overlays
- Teachers can assign grades to students (scoped to own subject)
- New tables: teacher_materials, teacher_announcements, teacher_exams, teacher_assigned_grades
- subject column added to users; included in JWT and /api/me
- requireTeacher middleware fetches fresh status+subject from DB on every request
- Login hint: username is the part of the school email before the @

index.js

@@ -3,6 +3,7 @@ const cookieParser = require('cookie-parser');
 const path = require('path');
 const routes = require('./src/routes');
 const { router: filesRouter } = require('./src/files');
+const teacherRouter = require('./src/teacher');
 
 const app = express();
 const PORT = 3010;
@@ -12,6 +13,7 @@ app.use(cookieParser());
 app.use(express.static(path.join(__dirname, 'public')));
 app.use('/api', routes);
 app.use('/api/files', filesRouter);
+app.use('/api/teacher', teacherRouter);
 
 const html = f => (req, res) => res.sendFile(path.join(__dirname, 'public', f));