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.

.github/workflows/ci.yml

@@ -51,4 +51,4 @@ jobs:
       # machine; CI runners have no CUDA toolkit and must build CPU-only.
       - name: Clippy
         working-directory: src-tauri
-        run: cargo clippy --all-targets --locked --no-default-features
+        run: cargo clippy --all-targets --locked --no-default-features -- -D warnings