Fix ?unclaimed filter to restrict to active-round specs only

[PunchTheDev]

Summary

GET /specs?unclaimed=true was returning 108 results (all specs with no passing submission, including inactive Thingiverse catalog entries) instead of the 42 active competition specs with no SOTA. Fixed by implicitly applying the active-round filter when unclaimed=True.

Changes

• app/routes/specs.py: unclaimed=True now implies active=True — restricts to specs whose round_id is in an active round. Non-competition specs (Thingiverse catalog, round_id=None) are never "claimable" in competition context.
• Tests updated: three unclaimed-filter tests now use a dedicated unclaimed_client fixture with SPECS_DIR=tests/fixtures/specs_round (contains r01_001_easy, which has round_id=round_001 via ID prefix) and ROUNDS_DIR=tests/fixtures/rounds_active.
• New fixture: tests/fixtures/specs_round/r01_001_easy.json.
• conftest.py _make_client() now also reloads app.specs to prevent module-level SPECS_DIR stale reads across tests.