Add worksheet short-answer grading view, student inbox, and parent-student messaging

- Grading queue now shows short-answer questions and student responses inline so the parent can read answers before scoring
- Student inbox tab shows all graded items with score, percentage, and parent feedback; unread badge appears on login
- Parent-student messaging: students send messages from the Inbox tab; Messages panel in admin shows threads with unread count badge, full conversation history, and reply compose

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

db.js

@@ -206,6 +206,15 @@ db.exec(`
     answers TEXT,
     completed_at TEXT NOT NULL DEFAULT (datetime('now'))
   );
+
+  CREATE TABLE IF NOT EXISTS messages (
+    id INTEGER PRIMARY KEY,
+    student_id INTEGER NOT NULL REFERENCES students(id) ON DELETE CASCADE,
+    sender TEXT NOT NULL,
+    body TEXT NOT NULL,
+    sent_at TEXT NOT NULL DEFAULT (datetime('now')),
+    read_at TEXT
+  );
 `);
 
 function tryAlter(sql) {