Fix: create_new_library existence check uses pre-safe-name string (#1155)

[shr8769]

Fixes #1155.

create_new_library() ran the database-level safe_name() conversion
(which rewrites characters like '-' to '_' on sqlite/postgres) after
check_if_library_exists(). Calling create_new_library with the same
original name a second time checked existence under the un-rewritten
name, found nothing, and attempted to recreate the library — colliding
with the already-registered remapped name and raising an
IntegrityError.

This moves the safe_name conversion before the existence check, so
both operate on the same final name. Added a regression test
(test_create_new_library_idempotent_with_unsafe_name) that reproduces
the original failure on sqlite and confirms the fix.

Verified locally:

• Before the fix: second create_new_library() call with the same
  unsafe name raised sqlite3.IntegrityError.
• After the fix: both calls succeed and resolve to the same library.
• New regression test passes: 1 passed.