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/media.rs

@@ -49,11 +49,7 @@ impl MediaTools {
                 total_bytes,
                 downloaded_bytes,
             } => {
-                log::info!(
-                    "Downloading bundled FFmpeg: {}/{} bytes",
-                    downloaded_bytes,
-                    total_bytes
-                );
+                log::info!("Downloading bundled FFmpeg: {downloaded_bytes}/{total_bytes} bytes");
             }
             FfmpegDownloadProgressEvent::UnpackingArchive => {
                 log::info!("Unpacking bundled FFmpeg...");