feat(watcher): adaptive filesystem watchdog with zero CPU when idle

Monitors all registered folders using OS-native events (ReadDirectoryChangesW
on Windows) so new, modified, and deleted files are reflected in the gallery
automatically without manual reindexing.

Key design points:
- Adaptive blocking: recv() when no events pending (zero CPU), switches to
  recv_timeout(earliest_deadline) only when debounce timers are running — wakes
  exactly when the soonest event is ready, no busy-polling
- 500 ms per-path debounce coalesces rapid OS event bursts into one action
- Change detection preserved: build_record skips upsert if file_size + mtime
  unchanged, preventing spurious thumbnail/embedding re-queues and avoiding
  clobbering of existing metadata like thumbnail_path
- Access events (reads) filtered out; only Create/Modify/Remove trigger work
- Deletion path: emits watcher-deleted event with image IDs; frontend removes
  those images from state and clears selectedImage if it was deleted

WatcherHandle stored in app state; add_folder / remove_folder / update_folder_path
commands keep the watched path set in sync with the DB.

src-tauri/src/db.rs

@@ -1352,6 +1352,44 @@ pub fn update_image_details(
     .map_err(Into::into)
 }
 
+/// Look up the lightweight indexed-media entry for a single path.
+/// Used by the filesystem watcher to run change-detection before upserting.
+pub fn get_indexed_entry_by_path(conn: &Connection, path: &str) -> Result<Option<IndexedMediaEntry>> {
+    let result = conn.query_row(
+        "SELECT id, path, modified_at, file_size, media_kind FROM images WHERE path = ?1",
+        [path],
+        |row| {
+            Ok(IndexedMediaEntry {
+                id: row.get(0)?,
+                path: row.get(1)?,
+                modified_at: row.get(2)?,
+                file_size: row.get(3)?,
+                media_kind: row.get(4)?,
+            })
+        },
+    );
+    match result {
+        Ok(entry) => Ok(Some(entry)),
+        Err(rusqlite::Error::QueryReturnedNoRows) => Ok(None),
+        Err(e) => Err(e.into()),
+    }
+}
+
+/// Look up just the image id for a path. Used by the filesystem watcher
+/// to find the DB row to delete when a file is removed from disk.
+pub fn get_image_id_by_path(conn: &Connection, path: &str) -> Result<Option<i64>> {
+    let result = conn.query_row(
+        "SELECT id FROM images WHERE path = ?1",
+        [path],
+        |row| row.get(0),
+    );
+    match result {
+        Ok(id) => Ok(Some(id)),
+        Err(rusqlite::Error::QueryReturnedNoRows) => Ok(None),
+        Err(e) => Err(e.into()),
+    }
+}
+
 pub fn get_image_by_id(conn: &Connection, image_id: i64) -> Result<ImageRecord> {
     conn.query_row(
         "SELECT id, folder_id, path, filename, thumbnail_path, width, height, file_size, created_at, modified_at, taken_at, mime_type,