indexer: apply EXCLUDED_DIRS to remaining WalkDir callers

Audit follow-up to 5bf4956. The same `@eaDir` pruning that protects
the indexer also needs to protect the other walks under library roots:

- `create_thumbnails` walks every file in every library to generate
  thumbnails. Without EXCLUDED_DIRS, it would generate thumbnails of
  Synology's `SYNOFILE_THUMB_*.jpg` thumbnails (thumbnails of thumbnails).
- `update_media_counts` walks for the prometheus IMAGE / VIDEO gauges.
  Without EXCLUDED_DIRS, the gauges over-count by however many phantom
  `@eaDir` images live alongside the real photos.
- `cleanup_orphaned_playlists` walks BASE_PATH searching for source
  videos by filename. EXCLUDED_DIRS isn't a behavior change for typical
  Synology mounts (no .mp4 in @eaDir), but it's a correctness win for
  any operator-defined exclude that happens to contain video.

Refactor: add `walk_library_files(base, excluded_dirs) -> Vec<DirEntry>`
to file_scan.rs as the shared primitive. `enumerate_indexable_files`
now layers media-type + mtime filters on top of it. One new test
covers the lower-level helper (returns all extensions, prunes excluded
subtrees).

`generate_video_gifs` (currently `#[allow(dead_code)]`, not reachable
from main) gets the `update_media_counts` signature update and reads
EXCLUDED_DIRS from env so a future revival isn't broken — but its
WalkDir walk stays raw because the dual lib/bin compile makes the
file_scan module path non-trivial there. Tagged with a comment.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib.rs

@@ -40,11 +40,11 @@ pub use state::AppState;
 use std::path::Path;
 use walkdir::DirEntry;
 
-pub fn create_thumbnails(_libs: &[libraries::Library]) {
+pub fn create_thumbnails(_libs: &[libraries::Library], _excluded_dirs: &[String]) {
     // Stub - implemented in main.rs
 }
 
-pub fn update_media_counts(_media_dir: &Path) {
+pub fn update_media_counts(_media_dir: &Path, _excluded_dirs: &[String]) {
     // Stub - implemented in main.rs
 }