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/embedder.rs

@@ -160,7 +160,7 @@ impl ClipImageEmbedder {
             let ids = enc.get_ids();
             let len = ids.len().min(max_len);
             for j in 0..len {
-                flat[i * max_len + j] = ids[j] as u32;
+                flat[i * max_len + j] = ids[j];
             }
         }