Let streamers set how long messages stay on screen

Adds display_duration_seconds to the streamers table (default 10s,
with an idempotent migration for existing rows), a dashboard control
to change it, and wires the value through to the overlay via the
display_message payload instead of a hardcoded 10s.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

db.js

@@ -18,6 +18,7 @@ db.exec(`
     overlay_token TEXT UNIQUE NOT NULL,
     sender_token TEXT UNIQUE NOT NULL,
     tts_voice TEXT NOT NULL DEFAULT 'ru-RU-DmitryNeural',
+    display_duration_seconds INTEGER NOT NULL DEFAULT 10,
     created_at INTEGER NOT NULL
   );
 
@@ -30,6 +31,12 @@ db.exec(`
   );
 `);
 
+// Idempotent migration for databases created before display_duration_seconds existed.
+const existingColumns = db.prepare('PRAGMA table_info(streamers)').all().map((c) => c.name);
+if (!existingColumns.includes('display_duration_seconds')) {
+  db.exec('ALTER TABLE streamers ADD COLUMN display_duration_seconds INTEGER NOT NULL DEFAULT 10');
+}
+
 function generateToken() {
   return crypto.randomBytes(16).toString('hex');
 }
@@ -75,10 +82,15 @@ function setViewerName(streamerId, googleId, displayName) {
   ).run(streamerId, googleId, displayName, Date.now());
 }
 
+function updateDisplayDuration(streamerId, seconds) {
+  db.prepare('UPDATE streamers SET display_duration_seconds = ? WHERE id = ?').run(seconds, streamerId);
+}
+
 module.exports = {
   findStreamerByOverlayToken,
   findStreamerBySenderToken,
   getOrCreateStreamerByGoogle,
   getViewerName,
   setViewerName,
+  updateDisplayDuration,
 };