feat(audio): add offline local neural voice via Piper (sv + en)

Announcements can use Piper, a free local neural TTS, instead of the browser's system
voice. No account, no paid service, and it keeps working with no internet at the track.

- npm run tts:install (or Settings -> Audio -> Install) downloads the standalone Piper
  release for Linux/Windows/macOS plus Swedish and English voices into piper/ once
- server/tts.js generates each line once, one at a time, and caches it as WAV under
  data/tts/; a new line takes ~0.3 s with the standalone build
- src/piper_speech.js plays it and falls back to the browser voice if Piper is missing,
  errors, or takes over 2.5 s, so no announcement is lost and timing is never held up
- "Pre-generate phrases and driver names" renders every fixed phrase and name in both
  languages before race day

Browser voice stays the default. Generation routes fall under the admin PIN.

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

scripts/install-piper.js

@@ -0,0 +1,13 @@
+// Installs local neural voices for race announcements: the standalone Piper release for
+// this OS plus Swedish and English voices, into piper/. Run once with internet access:
+//   npm run tts:install
+const path = require("path");
+const { resolvePiperBinary } = require("../server/tts");
+const { installAll } = require("../server/tts_install");
+
+const projectDir = path.join(__dirname, "..");
+installAll({ projectDir, hasBinary: Boolean(resolvePiperBinary(process.env.RC_PIPER_BIN || "", projectDir)), log: (line) => console.log(line) })
+  .catch((error) => {
+    console.error(`Piper install failed: ${error.message}`);
+    process.exit(1);
+  });