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-audio/Cargo.toml

@@ -0,0 +1,25 @@
+[package]
+name = "qbz-audio"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+description = "Audio backend system for QBZ - bit-perfect playback"
+
+[dependencies]
+qbz-models = { path = "../qbz-models" }
+
+# Async
+tokio = { workspace = true }
+async-trait = { workspace = true }
+
+# Logging
+log = { workspace = true }
+
+# Error handling
+thiserror = { workspace = true }
+
+# Audio will use vendored rodio/cpal from qbz-player
+# symphonia = { workspace = true }
+
+[target.'cfg(target_os = "linux")'.dependencies]
+alsa = "0.9"