When the year-suffixed target folder already exists on disk, both
FolderNamingService and AnimeService.rename_folder_if_needed used to
silently bail out. The bare folder (e.g. 'Ultraman') was left next to
the year-suffixed one ('Ultraman (2019)'), producing the symptom
'reports series like Ultraman as added twice' — the DB has one row but
the filesystem has two folders holding the same content.
Fix: when the target already exists, merge the source's contents into
the target (target version wins on file conflicts; source copies are
removed so cleanup succeeds), remove the now-empty source directory,
update DB row + in-memory cache. Plain rename path is unchanged.
Also fixes a latent TypeError in rename_folder_if_needed where
self._directory (a str) was used with the '/' operator. Production
behavior was that any rename through that method raised and was
swallowed by the caller's try/except, leaving the bare folder
untouched. The new path builds Path objects from the string base.
Tests:
- Replaced test_skips_when_target_folder_already_exists (which
codified the bug) with three tests that cover the new merge
contract: clean merge, no-overwrite, and empty-source removal.
- Added tests/unit/test_rename_folder_if_needed.py covering the
same scenarios plus the str-directory regression. All seven go
red on the unfixed code and green with the fix.
Fixes the 'Ultraman' / 'Ultraman (2019)' duplicate-folder bug.
