Add spelling improvements and local Piper TTS

Practice: fire streak badge, first-letter hint on wrong answers, missed-words drill session after practice completes.
Test: audio-only mode with listening state before input appears, larger re-read button.
Words: definition field (DB migration + admin editor + TTS playback).
TTS: replace speechSynthesis with local Piper via /api/tts, WAV cached in data/tts-cache, falls back to browser voice when Piper is not installed.
Dockerfile: switched to node:26-slim, installs Piper binary + en_US-lessac-medium voice.

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

db.js

@@ -222,6 +222,7 @@ tryAlter(`ALTER TABLE submissions ADD COLUMN student_note TEXT`);
 tryAlter(`ALTER TABLE submissions ADD COLUMN parent_comment TEXT`);
 tryAlter(`ALTER TABLE schedule ADD COLUMN status TEXT NOT NULL DEFAULT 'not_started'`);
 tryAlter(`ALTER TABLE schedule ADD COLUMN evidence_notes TEXT`);
+tryAlter(`ALTER TABLE words ADD COLUMN definition TEXT NOT NULL DEFAULT ''`);
 tryAlter(`ALTER TABLE schedule ADD COLUMN evidence_photo TEXT`);
 
 export const sha256 = (s) => createHash('sha256').update(String(s)).digest('hex');