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/video/ffmpeg.rs

@@ -40,7 +40,10 @@ pub struct Ffmpeg;
 
 pub enum GifType {
     Overview,
-    OverviewVideo { duration: u32 },
+    #[allow(dead_code)]
+    OverviewVideo {
+        duration: u32,
+    },
 }
 
 impl Ffmpeg {