feat(spotify): add SpotifyAudioBackend interface + go-librespot config.yml renderer

- backend.ts: type-only SpotifyAudioBackend contract + track/metadata DTOs
- go-librespot-config.ts: renderConfigYml() hand-built config (pipe/s16le,
  server enabled, interactive OAuth), no yaml dependency
- tests assert exact keys/values and round-trip via a tiny structural parser

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/music/spotify/go-librespot-config.ts

@@ -0,0 +1,42 @@
+// src/music/spotify/go-librespot-config.ts
+// Hand-built go-librespot config.yml. Keys/values verified against
+// devgianlu/go-librespot cmd/daemon/cli_config.go koanf tags. No yaml
+// dependency is used; the file is a small, fixed-shape document.
+
+export interface GoLibrespotConfigOptions {
+  deviceName: string;
+  bitrate: number;
+  fifoPath: string;
+  apiAddress: string;
+  apiPort: number;
+  callbackPort: number;
+}
+
+/**
+ * Render a headless go-librespot config.yml:
+ *  - pipe audio backend writing raw 44.1kHz/s16le stereo PCM to a FIFO,
+ *  - HTTP+WebSocket control server enabled (port has NO built-in default,
+ *    so it is always written explicitly),
+ *  - interactive OAuth credentials (persisted automatically to
+ *    <config_dir>/credentials.json after first login).
+ */
+export function renderConfigYml(o: GoLibrespotConfigOptions): string {
+  return (
+    [
+      `device_name: ${JSON.stringify(o.deviceName)}`,
+      `device_type: computer`,
+      `bitrate: ${o.bitrate}`,
+      `audio_backend: pipe`,
+      `audio_output_pipe: ${o.fifoPath}`,
+      `audio_output_pipe_format: s16le`,
+      `server:`,
+      `  enabled: true`,
+      `  address: ${o.apiAddress}`,
+      `  port: ${o.apiPort}`,
+      `credentials:`,
+      `  type: interactive`,
+      `  interactive:`,
+      `    callback_port: ${o.callbackPort}`,
+    ].join("\n") + "\n"
+  );
+}