test(conftest): migrate test_db.py local shadow card fixtures to uuid4()

[ImmortalDemonGod]

What

In tests/test_db.py, three local fixture implementations shadow the conftest.py card fixtures with the same constant UUID literals that F8 replaced in conftest:

• Line 105: uuid.UUID("11111111-1111-1111-1111-111111111111")
• Line 124: uuid.UUID("22222222-2222-2222-2222-222222222222")
• Line 143: uuid.UUID("33333333-3333-3333-3333-333333333333")

Why deferred

F8 (PR fix/flashcore-F8) brought uuid4() into scope only for the shared conftest.py card fixtures (sample_card1, sample_card2, sample_card3_deck_b). The test_db.py shadow fixtures are independent — they are local to test_db.py and do not depend on conftest's UUID values, so changing them was outside F8's scope per plan §6: "Migrating test_db.py local shadow fixtures to uuid4() — each file's fixture convention is internally consistent; F8 only mandates conftest card fixtures." Classification: nice-to-have.

Acceptance criteria

• tests/test_db.py local shadow fixtures for sample_card1, sample_card2, sample_card3_deck_b use uuid.uuid4() (or str(uuid.uuid4())) instead of hardcoded literal strings.
• pytest tests/ -q --tb=short exits 0 with passed count ≥ 499, 1 skipped.
• No other test file is broken by the change (conftest consumers and test_db.py are independent; the shadow fixtures only affect test_db.py scope).

References

• Plan §6 deferred scope: .aiv/plans/flashcore-F8-plan.md §6
• Canonical audit source (F8): https://github.com/ImmortalDemonGod/flashcore/blob/fb1ae5a1c1893939f4ff4f82cbd09d4e90f8e965/audit/02-static-audit.md#L18