refactor: DDD/CQRS architecture, unified crate layout

- crates: common→application, api→presentation, infrastructure/*→adapters/*
- new crates: api-types, infra-wiring
- domain: errors/, models/, value_objects/, ports/, services/
- application: CQRS use cases (songs/, tabs/) w/ commands, queries, deps
- unified DomainError replaces RepositoryError
- workspace deps, unused dep cleanup
- fix: parse plain-text chord lines (UG drops spans mid-song)
- tests extracted to separate modules (tests/ dirs)

crates/application/src/songs/deps.rs

@@ -0,0 +1,11 @@
+use domain::ports::{SongRepositoryPort, SongSearchPort};
+use std::sync::Arc;
+
+pub struct SongCommandDeps {
+    pub repo: Arc<dyn SongRepositoryPort>,
+}
+
+pub struct SongQueryDeps {
+    pub repo: Arc<dyn SongRepositoryPort>,
+    pub search: Arc<dyn SongSearchPort>,
+}