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/api-types/Cargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "api-types"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+serde = { workspace = true }

crates/api-types/src/lib.rs

@@ -0,0 +1,31 @@
+use serde::{Deserialize, Serialize};
+
+#[derive(Deserialize)]
+pub struct ParseRequest {
+    pub source: Option<String>,
+    pub html: Option<String>,
+}
+
+#[derive(Serialize)]
+pub struct ErrorResponse {
+    pub error: String,
+}
+
+#[derive(Deserialize)]
+pub struct ListQuery {
+    pub q: Option<String>,
+    pub sort: Option<String>,
+    pub order: Option<String>,
+}
+
+#[derive(Deserialize)]
+pub struct UpdateSongRequest {
+    pub title: Option<String>,
+    pub artist: Option<String>,
+    pub original_key: Option<String>,
+}
+
+#[derive(Deserialize)]
+pub struct GetSongQuery {
+    pub apply_capo: Option<bool>,
+}