Give backup album folders distinct names and remove stale entries (closes #103)

Within a collection's folder, files whose sanitized titles match (ignoring
case) each get their file ID added before the extension, and collections
whose sanitized names match get their ID added, so no symlink or JSON
replaces another. Names are chosen across all collections, so a scoped run
names folders the same as a full one.

Each run first removes symlinks into originals/ that no longer belong to a
collection, and the folders quak wrote (a sibling JSON with an album ID) for
collections that are gone or renamed. Anything else is left alone; a folder
still holding user files keeps its JSON.

Model: opus-5-5

TODO.md

@@ -18,6 +18,13 @@ Tag v1.0.0.
 
 # Completed Steps
 
+- 2026-09-23: Fixed the backup's per-collection folders (issue 103). Two files
+  in one collection with the same title, and two collections with the same name,
+  each get their ID added to the name (`IMG_0001 (12345).JPG`, `Trip (10)/`), so
+  none replaces another's symlink or JSON. Each run removes symlinks into
+  `originals/` for files no longer in the collection, and the folders of deleted
+  or renamed collections, leaving anything else in `collections/` alone. The
+  README backup layout states the naming rule.
 - 2026-09-23: Kept one account's cache from mixing with another's (issue 104).
   When `metadata.json` in the cache directory was written for a different,
   non-zero user ID than the client's, `Library.open` deletes it and `mldata/`