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/storage.rs

@@ -1,4 +1,4 @@
-use std::path::{Path, PathBuf};
+use std::path::Path;
 use sysinfo::{DiskKind, Disks};
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -107,7 +107,7 @@ pub fn detect_storage_profile(path: &Path) -> StorageProfile {
     }
 }
 
-fn fallback_profile_for_path(path: &PathBuf) -> StorageProfile {
+fn fallback_profile_for_path(path: &Path) -> StorageProfile {
     let path_str = path.to_string_lossy().to_lowercase();
     if path_str.starts_with("\\\\") {
         return StorageProfile::Conservative;