Let viewers set a per-streamer display name

Viewers can type a custom name shown to the streamer instead of
their Google account name. It's saved per (streamer, viewer) pair
in SQLite and pre-filled on future visits to the same send link.

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

db.js

@@ -20,6 +20,14 @@ db.exec(`
     tts_voice TEXT NOT NULL DEFAULT 'ru-RU-DmitryNeural',
     created_at INTEGER NOT NULL
   );
+
+  CREATE TABLE IF NOT EXISTS viewer_names (
+    streamer_id INTEGER NOT NULL,
+    google_id TEXT NOT NULL,
+    display_name TEXT NOT NULL,
+    updated_at INTEGER NOT NULL,
+    PRIMARY KEY (streamer_id, google_id)
+  );
 `);
 
 function generateToken() {
@@ -52,8 +60,25 @@ function getOrCreateStreamerByGoogle({ googleId, email, name, picture }) {
   return findStreamerByGoogleId(googleId) || createStreamer({ googleId, email, name, picture });
 }
 
+function getViewerName(streamerId, googleId) {
+  const row = db
+    .prepare('SELECT display_name FROM viewer_names WHERE streamer_id = ? AND google_id = ?')
+    .get(streamerId, googleId);
+  return row ? row.display_name : null;
+}
+
+function setViewerName(streamerId, googleId, displayName) {
+  db.prepare(
+    `INSERT INTO viewer_names (streamer_id, google_id, display_name, updated_at)
+     VALUES (?, ?, ?, ?)
+     ON CONFLICT(streamer_id, google_id) DO UPDATE SET display_name = excluded.display_name, updated_at = excluded.updated_at`
+  ).run(streamerId, googleId, displayName, Date.now());
+}
+
 module.exports = {
   findStreamerByOverlayToken,
   findStreamerBySenderToken,
   getOrCreateStreamerByGoogle,
+  getViewerName,
+  setViewerName,
 };