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/domain/Cargo.toml

@@ -4,10 +4,7 @@ version = "0.1.0"
 edition = "2024"
 
 [dependencies]
-anyhow = { workspace = true }
 thiserror = { workspace = true }
-tracing = { workspace = true }
 uuid = { workspace = true }
-rand = { workspace = true }
 serde = { workspace = true }
 async-trait = { workspace = true }