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/backend.ts

@@ -0,0 +1,41 @@
+// src/music/spotify/backend.ts
+// Type contract for the Spotify audio backend (go-librespot sidecar).
+// Interface-only: this module intentionally contains NO runtime code so it
+// can be imported for types by go-librespot.ts and controller.ts without
+// pulling in child_process/ws/ffmpeg at type-check time.
+
+/** Emitted when the currently playing Spotify track finishes or is stopped. */
+export interface SpotifyTrackEndedEvent {
+  uri: string;
+  reason: "ended" | "stopped" | "error";
+}
+
+/** Now-playing metadata surfaced from the go-librespot "metadata" event. */
+export interface SpotifyNowPlaying {
+  uri: string;
+  name: string;
+  artist: string;
+  album: string;
+  coverUrl: string;
+  durationMs: number;
+}
+
+/**
+ * Long-lived Spotify audio source: owns the go-librespot sidecar + FIFO->ffmpeg
+ * PCM pipe and exposes transport control plus a continuous 48kHz s16le stereo
+ * PCM stream to feed AudioPlayer.playPcmStream().
+ */
+export interface SpotifyAudioBackend {
+  start(): Promise<void>;
+  stop(): void;
+  isReady(): boolean;
+  playTrack(uri: string): Promise<void>;
+  pause(): Promise<void>;
+  resume(): Promise<void>;
+  seek(ms: number): Promise<void>;
+  getPcmStream(): import("node:stream").Readable;
+  getPositionMs(): number;
+  on(event: "trackEnded", cb: (e: SpotifyTrackEndedEvent) => void): void;
+  on(event: "metadata", cb: (m: SpotifyNowPlaying) => void): void;
+  on(event: "ready" | "error", cb: (arg?: unknown) => void): void;
+}