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/testhelpers.rs

@@ -14,6 +14,12 @@ pub struct TestUserDao {
     pub user_map: RefCell<Vec<User>>,
 }
 
+impl Default for TestUserDao {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl TestUserDao {
     pub fn new() -> Self {
         Self {
@@ -71,6 +77,12 @@ pub struct TestPreviewDao {
     next_id: StdMutex<i32>,
 }
 
+impl Default for TestPreviewDao {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl TestPreviewDao {
     pub fn new() -> Self {
         Self {