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/video/mod.rs

@@ -24,6 +24,17 @@ pub async fn generate_video_gifs() {
         fs::create_dir_all(gif_base_path).expect("There was an issue creating directory");
 
         let files = PathBuf::from(dotenv::var("BASE_PATH").unwrap());
+        // EXCLUDED_DIRS read here for the update_media_counts call below.
+        // The WalkDir walk itself is left raw — this function is currently
+        // dead code (`#[allow(dead_code)]`) and not reachable from main.
+        // If revived, swap to file_scan::walk_library_files (dual lib/bin
+        // module path makes that non-trivial here).
+        let excluded_dirs: Vec<String> = std::env::var("EXCLUDED_DIRS")
+            .unwrap_or_default()
+            .split(',')
+            .filter(|s| !s.trim().is_empty())
+            .map(|s| s.trim().to_string())
+            .collect();
 
         let ffmpeg = Ffmpeg;
         for file in WalkDir::new(&files)
@@ -62,6 +73,6 @@ pub async fn generate_video_gifs() {
 
         info!("Finished making video gifs in {:?}", start.elapsed());
 
-        update_media_counts(&files);
+        update_media_counts(&files, &excluded_dirs);
     });
 }