Generate a session secret instead of refusing to start

setup.sh starts the production server without SESSION_SECRET, so the new
"refuse to start" check stopped the app after an update. When the variable
is unset in production the server now makes a random secret once and keeps
it in DATA_DIR/.session-secret (mode 600, git-ignored).

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

server/index.js

@@ -2,11 +2,12 @@ import express from 'express';
 import compression from 'compression';
 import session from 'express-session';
 import QRCode from 'qrcode';
-import { randomUUID } from 'crypto';
+import { randomUUID, randomBytes } from 'crypto';
+import { existsSync, mkdirSync, readFileSync, writeFileSync } from 'fs';
 import { fileURLToPath } from 'url';
 import { dirname, join } from 'path';
 import {
-  initDb, getAllProjects, getProject, getProjectEdited, upsertProject, deleteProject,
+  initDb, dataDir, getAllProjects, getProject, getProjectEdited, upsertProject, deleteProject,
   countUsers, createUser, getUserByEmail, getUserById, claimOrphanProjects,
   enableTotp, disableTotp, setBackupCodeHashes, setPodcastName,
   getShareToken, setShareToken, clearShareToken, getProjectByShareToken,
@@ -50,9 +51,23 @@ const app = express();
 const PORT = process.env.PORT || 3001;
 const isProd = process.env.NODE_ENV === 'production';
 
-if (isProd && !process.env.SESSION_SECRET) {
-  console.error('SESSION_SECRET is not set. Set it to a long random string before running in production.');
-  process.exit(1);
+// Session cookies are signed with SESSION_SECRET. When it isn't set (setup.sh doesn't set one), a random secret
+// is made once and kept in DATA_DIR, so it survives restarts and nobody has to configure anything.
+function sessionSecret() {
+  if (process.env.SESSION_SECRET) return process.env.SESSION_SECRET;
+  if (!isProd) return 'dev-only-secret-change-me';
+  const file = join(dataDir(), '.session-secret');
+  try {
+    if (existsSync(file)) return readFileSync(file, 'utf8').trim();
+    mkdirSync(dataDir(), { recursive: true });
+    const made = randomBytes(48).toString('hex');
+    writeFileSync(file, made, { mode: 0o600 });
+    console.log(`SESSION_SECRET is not set; generated one and saved it to ${file}.`);
+    return made;
+  } catch (err) {
+    console.error('Could not read or save a session secret; set SESSION_SECRET instead.', err.message);
+    process.exit(1);
+  }
 }
 
 // Trust the reverse proxy (needed for secure cookies to work behind nginx/etc).
@@ -98,7 +113,7 @@ app.use(compression());
 app.use(express.json({ limit: '10mb' }));
 app.use(session({
   store: new SqliteSessionStore(),
-  secret: process.env.SESSION_SECRET || 'dev-only-secret-change-me',
+  secret: sessionSecret(),
   resave: false,
   saveUninitialized: false,
   rolling: true,