Add streamer dashboard stats; fix TTS overlap race properly

Dashboard now shows unique visitors (via sender link), total messages,
and unique senders per streamer, tracked via new viewer_visits/messages
tables.

The previous overlay TTS-overlap fix had a race: audio not having
arrived yet was treated the same as "no audio at all", letting the
queue advance before slow-to-synthesize audio (e.g. a long question)
ever started playing. Now "not arrived yet" is explicitly distinct
from "finished playing", with a grace-period fallback for genuine TTS
failures.

Also drop the hardcoded AI name from ai-prompt.txt since it's now
injected dynamically from the streamer's configured ai_name.

db.js

@@ -30,6 +30,21 @@ db.exec(`
     updated_at INTEGER NOT NULL,
     PRIMARY KEY (streamer_id, google_id)
   );
+
+  CREATE TABLE IF NOT EXISTS viewer_visits (
+    streamer_id INTEGER NOT NULL,
+    google_id TEXT NOT NULL,
+    first_seen_at INTEGER NOT NULL,
+    last_seen_at INTEGER NOT NULL,
+    PRIMARY KEY (streamer_id, google_id)
+  );
+
+  CREATE TABLE IF NOT EXISTS messages (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    streamer_id INTEGER NOT NULL,
+    google_id TEXT NOT NULL,
+    created_at INTEGER NOT NULL
+  );
 `);
 
 // Idempotent migration for databases created before display_duration_seconds existed.
@@ -94,6 +109,30 @@ function updateAiName(streamerId, aiName) {
   db.prepare('UPDATE streamers SET ai_name = ? WHERE id = ?').run(aiName, streamerId);
 }
 
+function recordVisit(streamerId, googleId) {
+  const now = Date.now();
+  db.prepare(
+    `INSERT INTO viewer_visits (streamer_id, google_id, first_seen_at, last_seen_at)
+     VALUES (?, ?, ?, ?)
+     ON CONFLICT(streamer_id, google_id) DO UPDATE SET last_seen_at = excluded.last_seen_at`
+  ).run(streamerId, googleId, now, now);
+}
+
+function recordMessage(streamerId, googleId) {
+  db.prepare(
+    'INSERT INTO messages (streamer_id, google_id, created_at) VALUES (?, ?, ?)'
+  ).run(streamerId, googleId, Date.now());
+}
+
+function getStreamerStats(streamerId) {
+  const visitors = db.prepare('SELECT COUNT(*) AS c FROM viewer_visits WHERE streamer_id = ?').get(streamerId).c;
+  const messages = db.prepare('SELECT COUNT(*) AS c FROM messages WHERE streamer_id = ?').get(streamerId).c;
+  const uniqueSenders = db
+    .prepare('SELECT COUNT(DISTINCT google_id) AS c FROM messages WHERE streamer_id = ?')
+    .get(streamerId).c;
+  return { visitors, messages, uniqueSenders };
+}
+
 module.exports = {
   findStreamerByOverlayToken,
   findStreamerBySenderToken,
@@ -102,4 +141,7 @@ module.exports = {
   setViewerName,
   updateDisplayDuration,
   updateAiName,
+  recordVisit,
+  recordMessage,
+  getStreamerStats,
 };