refactor(backend): zero clippy warnings, gate CI with -D warnings

Auto-fix pass (uninlined format args, needless borrows, to_vec,
size_of_val, map_err->inspect_err) plus hand fixes: fully annotated the
sqlite-vec transmute (c_char for cross-platform correctness), type alias
for the duplicate-scan tuple, slice signatures over &mut Vec/&PathBuf,
and #[allow(dead_code)] documenting the intentionally dormant caption
worker. The CI clippy step now fails on any new warning.

src-tauri/src/tagger.rs

@@ -481,7 +481,7 @@ impl WdTagger {
             .try_extract_tensor::<f32>()
             .map_err(|error| anyhow::anyhow!("{error}"))?;
 
-        let probs: &[f32] = &probabilities;
+        let probs: &[f32] = probabilities;
 
         if probs.len() != self.labels.len() {
             anyhow::bail!(