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/adapters/sqlite/migrations/001_songs.sql

@@ -0,0 +1,9 @@
+CREATE TABLE IF NOT EXISTS songs (
+    id             TEXT PRIMARY KEY,
+    title          TEXT NOT NULL,
+    artist         TEXT NOT NULL,
+    original_key   TEXT,
+    preview_chords TEXT NOT NULL,
+    body           TEXT NOT NULL,
+    created_at     TEXT NOT NULL DEFAULT (datetime('now'))
+);