refactor(backend): Phase 0 - Initialize multi-crate workspace

Create the foundational workspace structure for the multi-crate
architecture refactor. This phase establishes placeholder crates
that will be populated in subsequent phases.

Crates created:
- qbz-models: Shared types, events, and traits (Tier 0)
- qbz-audio: Audio backend system (Tier 1)
- qbz-player: Playback engine and queue (Tier 1)
- qbz-qobuz: Qobuz API client (Tier 1)
- qbz-core: Orchestrator (Tier 3)

All crates compile successfully with placeholder implementations.
The workspace uses shared dependencies via workspace.dependencies.

cc

crates/qbz-player/src/lib.rs

@@ -0,0 +1,21 @@
+//! QBZ Player - Playback engine and queue management
+//!
+//! This crate provides:
+//! - Player: Main playback control
+//! - QueueManager: Track queue management
+//! - StreamingSource: HTTP audio streaming
+//! - PlaybackCache: In-memory prefetch cache
+//!
+//! The vendor/ directory contains modified versions of:
+//! - rodio: High-level audio playback
+//! - cpal: Cross-platform audio I/O
+
+// TODO: Phase 3 - Copy player modules from qbz-nix
+// pub mod player;
+// pub mod queue;
+// pub mod streaming_source;
+// pub mod playback_engine;
+// pub mod playback_cache;
+
+// pub use player::Player;
+// pub use queue::QueueManager;