* Fix backup import dropping a user's skill on cross-tenant title/id collision
The skills block of import_data deduped incoming skills against
skills_manager.load_all(), which returns EVERY tenant's skills. So when
a user imports their own backup, any skill whose id or title collides
with another user's skill was silently skipped — the importing user
lost their own data. This is the same cross-tenant bug already fixed
for the memories block just above (#1743); the skills block was left
with the old pattern. Filter the dedup sets to the importing user's own
skills (owner == user); the full store is still saved back, preserving
other users' skills.
* Restore sys.modules after stubbing so backup test does not break collection of later src.* test modules
* Patch backup_routes auth helpers via monkeypatch instead of sys.modules stubs so the test is import-order robust
* Give FakeSkillsManager an add_skill method matching the disk-backed skills API
Afonso Coutinho