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/content_hash.rs

@@ -53,13 +53,18 @@ pub fn thumbnail_path(thumbs_dir: &Path, hash: &str) -> PathBuf {
 /// Hash-keyed HLS output directory: `<video_dir>/<hash[..2]>/<hash>/`.
 /// The playlist lives at `playlist.m3u8` inside this directory and its
 /// segments are co-located so HLS relative references Just Work.
+#[allow(dead_code)]
 pub fn hls_dir(video_dir: &Path, hash: &str) -> PathBuf {
     let shard = shard_prefix(hash);
     video_dir.join(shard).join(hash)
 }
 
 fn shard_prefix(hash: &str) -> &str {
-    let end = hash.char_indices().nth(2).map(|(i, _)| i).unwrap_or(hash.len());
+    let end = hash
+        .char_indices()
+        .nth(2)
+        .map(|(i, _)| i)
+        .unwrap_or(hash.len());
     &hash[..end]
 }