Remove unreliable duplicate-lesson-file matching

Filename-similarity matching within a folder can't distinguish a real
re-download duplicate from a course that splits one topic across
several numbered files ("...Part 1"/"...Part 2", "-1"/"-2"/"-3") -
both look like near-100% matches by name alone, and the latter is a
common, completely normal pattern. This produced hundreds of false
positives in practice. Removes Duplicate Lesson Files entirely
(detection function, its two API routes, and the File Management UI
section); whole-course-folder matching (Duplicate Courses) is
unaffected and stays the only duplicate scan in the app.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -185,20 +185,17 @@ silently stay blank instead of erroring.
   difference doesn't hide a real duplicate. Groups by similarity (union of
   any two courses over the threshold, transitively) rather than showing
   raw pairs; each course in a group gets a one-click Hide, or a **Delete**
-  that permanently removes it from disk (confirmed with the full path
-  before it runs). A group can also be marked **"Not a duplicate"** if the
-  match is wrong, which excludes that specific pair from future scans
-  without touching anything else that happens to match one of those
-  courses; ignored pairs are listed (and reversible) under "Ignored
-  matches," and persist through backup/restore alongside hidden paths and
-  Next Up.
-  - *Duplicate Lesson Files*: the same idea one level down - media files
-  sitting in the same folder that look like the same lesson downloaded
-  twice (e.g. a re-download that landed alongside the original instead of
-  replacing it). Compares only within a folder, at a higher match
-  threshold than Duplicate Courses, so two different lessons on a similar
-  topic don't get flagged; shares its ignore list with Duplicate Courses.
-  Delete here removes a single file, not a whole course.
+  that permanently removes it from disk (the one destructive action in the
+  whole app - confirmed with the full path before it runs). A group can
+  also be marked **"Not a duplicate"** if the match is wrong, which
+  excludes that specific pair from future scans without touching anything
+  else that happens to match one of those courses; ignored pairs are
+  listed (and reversible) under "Ignored matches," and persist through
+  backup/restore alongside hidden paths and Next Up. (Matching is
+  deliberately scoped to whole course folders, not individual lesson
+  files - courses that split one topic across several numbered files,
+  e.g. "...Part 1"/"...Part 2", look just as similar by name as a genuine
+  re-download, so file-level matching wasn't reliable enough to keep.)
   - *Clean Up Stale References*: finds entries in the hidden-paths list,
   Next Up queue, Favorites, or Recently Viewed history that point at a path
   no longer on disk - normally from renaming/moving/deleting a course