Split main.rs: extract backfill drains and thumbnails into modules

main.rs drops from 3542 → ~2930 lines by moving:

- src/backfill.rs (new): backfill_unhashed_backlog,
  backfill_missing_date_taken, backfill_missing_content_hashes,
  build_face_candidates, process_face_backlog. Now unit-tested for
  the first time — 5 tests covering cap behavior, library-id
  filtering, missing-on-disk skip, and the video/unhashed/scanned
  filters on face-candidate selection.

- src/thumbnails.rs (new): unsupported_thumbnail_sentinel,
  generate_image_thumbnail, create_thumbnails, update_media_counts,
  is_image, is_video, plus the IMAGE_GAUGE / VIDEO_GAUGE Prometheus
  metrics. Replaces the no-op stubs that used to live in lib.rs.
  4 new unit tests for the sentinel path math and the
  walker-counts-images-vs-videos smoke path.

Supporting:
- SqliteExifDao::from_shared (test-only) so an SqliteExifDao and
  SqliteFaceDao can share one in-memory connection — required to
  test build_face_candidates against the real join.
- files.rs / video/{mod,actors}.rs import from crate::thumbnails::*
  instead of the now-removed stubs in lib.rs.

cargo test --bin image-api: 325 passing (was 314).

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

src/lib.rs

@@ -32,6 +32,7 @@ pub mod state;
 pub mod tags;
 #[cfg(test)]
 pub mod testhelpers;
+pub mod thumbnails;
 pub mod utils;
 pub mod video;
 
@@ -39,20 +40,3 @@ pub mod video;
 pub use data::{Claims, ThumbnailRequest};
 pub use database::{connect, schema};
 pub use state::AppState;
-
-// Stub functions for modules that reference main.rs
-// These are not used by cleanup_files binary
-use std::path::Path;
-use walkdir::DirEntry;
-
-pub fn create_thumbnails(_libs: &[libraries::Library], _excluded_dirs: &[String]) {
-    // Stub - implemented in main.rs
-}
-
-pub fn update_media_counts(_media_dir: &Path, _excluded_dirs: &[String]) {
-    // Stub - implemented in main.rs
-}
-
-pub fn is_video(entry: &DirEntry) -> bool {
-    file_types::direntry_is_video(entry)
-}