faces: surface enabled/disabled state + per-tick candidate count

Manual deploy debugging: 'Saved thumbnail' logs were visible (boot-time
thumbnail backfill) but no face_watch logs were appearing, with no
obvious way to tell whether the integration was disabled, hadn't reached
a full scan yet, or had simply seen no new files.

Two log lines:
  - watch_files startup: 'Face detection: ENABLED' / 'DISABLED (set
    APOLLO_FACE_API_BASE_URL or APOLLO_API_BASE_URL to enable)' so
    you can tell at a glance whether the env wired through.
  - process_new_files (debug-level): 'face_watch: scan tick — N image
    file(s) walked, M candidate(s) (library 'main', modified_since=...)'
    so an empty-candidate scan is distinguishable from a misconfigured
    or skipped one without bumping log level for the rest of the
    watcher.

No behavior change.

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

src/main.rs

@@ -1810,6 +1810,18 @@ fn watch_files(
         info!("Starting optimized file watcher");
         info!("  Quick scan interval: {} seconds", quick_interval_secs);
         info!("  Full scan interval: {} seconds", full_interval_secs);
+        // Surface face-detection state at boot so it's obvious whether
+        // the watcher will hit Apollo. The branch silently no-ops when
+        // disabled (intentional for legacy deploys), which makes "why
+        // aren't faces being detected?" hard to diagnose otherwise.
+        if face_client.is_enabled() {
+            info!("  Face detection: ENABLED");
+        } else {
+            info!(
+                "  Face detection: DISABLED (set APOLLO_FACE_API_BASE_URL \
+                 or APOLLO_API_BASE_URL to enable)"
+            );
+        }
         for lib in &libs {
             info!(
                 "  Watching library '{}' (id={}) at {}",
@@ -2115,6 +2127,13 @@ fn process_new_files(
     // up; the watcher remains usable on legacy deploys.
     if face_client.is_enabled() {
         let candidates = build_face_candidates(&context, &files, &exif_dao, &face_dao);
+        debug!(
+            "face_watch: scan tick — {} image file(s) walked, {} candidate(s) (library '{}', modified_since={})",
+            files.iter().filter(|(p, _)| !is_video_file(p)).count(),
+            candidates.len(),
+            library.name,
+            modified_since.is_some(),
+        );
         if !candidates.is_empty() {
             face_watch::run_face_detection_pass(
                 library,