No way to create multiple short URLs in one request

A customer importing a spreadsheet of 500 URLs to shorten (a common onboarding flow for any link-management product) currently has to make 500 separate POST /api/urls calls, each with its own network round trip -- slow, and with no way to see a single, coherent "here is what happened to each of your 500 rows" result. Worse, any client naively looping over POST /api/urls and stopping on the first error loses no data, but a client that does not handle errors per-item carefully can misreport which specific items succeeded.

POST /api/urls/bulk has a route registered but it is a placeholder that raises NotImplementedError("POST /api/urls/bulk is not implemented yet (see TICKET-19)").

1. pytest practice_tickets/tests/test_ticket_19_bulk_create.py -v
2. Or manually: POST /api/urls/bulk with {"items": [...]}
3. Observe: NotImplementedError / 500, regardless of the body sent.

The "one bad item shouldn't sink the batch" requirement is the interesting part. If every item in the batch shares one outer DB transaction and any single IntegrityError aborts that whole transaction, every other item in the batch fails too (even ones that were perfectly valid) unless each item's insert is wrapped in its own recoverable unit of work -- e.g. a SQLAlchemy SAVEPOINT (db.session.begin_nested()) per item, so a failed item can be rolled back to that savepoint alone while the surrounding session/transaction (and everything already committed within it) stays intact.

Look at how a single create is validated and inserted in create_short_url -- the per-item logic should be very close to that, just looped. Look up db.session.begin_nested() for how to give each item its own rollback boundary within one request/session, so a collision on item 2 doesn't force you to redo (or lose) item 1's insert.