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/ug-parser/src/fetcher.rs

@@ -0,0 +1,52 @@
+use async_trait::async_trait;
+use domain::{FetchError, TabFetcherPort, TabSource};
+
+pub struct UgTabFetcher {
+    client: reqwest::Client,
+}
+
+impl UgTabFetcher {
+    pub fn new() -> Self {
+        Self {
+            client: reqwest::Client::new(),
+        }
+    }
+}
+
+impl Default for UgTabFetcher {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+#[async_trait]
+impl TabFetcherPort for UgTabFetcher {
+    async fn fetch(&self, source: TabSource) -> Result<String, FetchError> {
+        match source {
+            TabSource::File(path) => Ok(tokio::fs::read_to_string(&path).await?),
+            TabSource::Url(url) => {
+                let resp = self
+                    .client
+                    .get(&url)
+                    .send()
+                    .await
+                    .map_err(|e| FetchError::Network(e.to_string()))?;
+                let content_type = resp
+                    .headers()
+                    .get(reqwest::header::CONTENT_TYPE)
+                    .and_then(|v| v.to_str().ok())
+                    .unwrap_or("");
+                if !content_type.contains("text/html") {
+                    return Err(FetchError::InvalidContentType);
+                }
+                resp.text()
+                    .await
+                    .map_err(|e| FetchError::Network(e.to_string()))
+            }
+        }
+    }
+}
+
+#[cfg(test)]
+#[path = "tests/fetcher.rs"]
+mod tests;