chore: apply cargo fmt + clippy cleanup across crate

Silence forward-looking dead_code on unused DAO modules, annotate
individual placeholder items, rewrite tautological assert!(true/false)
in token tests as panic! arms, and pick up fmt drift.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/database/insights_dao.rs

@@ -31,6 +31,7 @@ pub trait InsightDao: Sync + Send {
         paths: &[String],
     ) -> Result<Option<PhotoInsight>, DbError>;
 
+    #[allow(dead_code)]
     fn get_insight_history(
         &mut self,
         context: &opentelemetry::Context,
@@ -79,6 +80,7 @@ impl SqliteInsightDao {
     }
 
     #[cfg(test)]
+    #[allow(dead_code)]
     pub fn from_connection(conn: Arc<Mutex<SqliteConnection>>) -> Self {
         SqliteInsightDao { connection: conn }
     }