LearnSpace: full-stack educational whiteboard platform

Node.js/Express backend + vanilla JS frontend.
Features: real-time collaborative whiteboard (SSE), multi-page support,
LaTeX formulas, shapes/connectors, coordinate systems, number lines,
compass, zoom/pan, Catmull-Rom pencil smoothing, ruler/protractor with
rotation & resize controls, minimap navigation overlay, auto-measurements,
multi-page thumbnails sidebar, PNG export, page templates.
Student/teacher workflows: classes, assignments, library, dashboard.
Mobile responsive. SQLite (better-sqlite3).

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

backend/src/db/db.js

@@ -0,0 +1,48 @@
+const { DatabaseSync } = require('node:sqlite');
+const path = require('path');
+
+const fs = require('fs');
+// Resolve DB_PATH: if absolute — use as-is; if relative — resolve from backend/ dir;
+// fallback to __dirname-relative default so CWD never matters.
+const _rawPath = process.env.DB_PATH;
+const _backendDir = path.join(__dirname, '../../');
+const dbPath = _rawPath
+  ? (path.isAbsolute(_rawPath) ? _rawPath : path.resolve(_backendDir, _rawPath))
+  : path.join(__dirname, '../../data/learnspace.db');
+const dbDir = path.dirname(dbPath);
+if (!fs.existsSync(dbDir)) fs.mkdirSync(dbDir, { recursive: true });
+
+// Auto-migrate from old location (backend/learnspace.db → backend/data/learnspace.db)
+const oldPath = path.join(__dirname, '../../learnspace.db');
+if (!fs.existsSync(dbPath) && fs.existsSync(oldPath)) {
+  fs.copyFileSync(oldPath, dbPath);
+  // Also copy WAL/SHM if present
+  if (fs.existsSync(oldPath + '-wal')) fs.copyFileSync(oldPath + '-wal', dbPath + '-wal');
+  if (fs.existsSync(oldPath + '-shm')) fs.copyFileSync(oldPath + '-shm', dbPath + '-shm');
+  console.log('[db] Migrated database from', oldPath, '→', dbPath);
+}
+
+const db = new DatabaseSync(dbPath);
+
+db.exec('PRAGMA journal_mode = WAL');
+db.exec('PRAGMA foreign_keys = ON');
+
+/**
+ * Run a synchronous function inside a BEGIN/COMMIT/ROLLBACK transaction.
+ * Returns the value returned by fn(), or rethrows on error.
+ */
+db.transaction = function transaction(fn) {
+  return (...args) => {
+    db.exec('BEGIN');
+    try {
+      const result = fn(...args);
+      db.exec('COMMIT');
+      return result;
+    } catch (err) {
+      db.exec('ROLLBACK');
+      throw err;
+    }
+  };
+};
+
+module.exports = db;