Improve error message when status database is locked (#720)

[chiliec]

Closes #720.

What

When the sqlite status database is locked (another vdirsyncer process running,
or a previous run interrupted), vdirsyncer sync printed a bare "Unknown error
occurred … Use -vdebug to see the full traceback." That's what the issue asked
to improve.

Fix

handle_cli_error now catches sqlite3.OperationalError. If the message is
database is locked, it prints an actionable hint (make sure no other
vdirsyncer process is running; delete the .items file for the collection as a
last resort). Any other OperationalError is re-raised so it still reaches the
generic handler unchanged.

Tests

Added to tests/system/cli/test_utils.py, mirroring the existing
test_handle_cli_error:

• test_handle_cli_error_database_locked — asserts the friendly message and no
  traceback.
• test_handle_cli_error_other_operational_error — asserts an unrelated
  OperationalError is re-raised.

Validation

$ python -m pytest tests/system/cli/test_utils.py -q
4 passed

RED→GREEN: with the source fix reverted (tests kept), both new tests fail
(assert 'status database is locked' in … fails; DID NOT RAISE OperationalError); with the fix, all pass.

ruff format --check clean on both files. Happy to adjust the wording.

[chiliec]

CI note: the three builds.sr.ht checks are red for infra reasons unrelated to this change.

• tests-minimal / tests-pypi error on every external-fork PR here (they can't run for forks) — e.g. add token command #1220/feat(google): add token_command support for external token fetching #1221 show the same two red.
• archlinux-py313 failed while installing a runtime dependency from the AUR, before any vdirsyncer test ran: error making: python-click-threading exit status 4 -> Failed to install the following packages. Manual intervention is required: python-click-threading.

codecov/project and docs/readthedocs.org are green; codecov reports +0.05% coverage from the two added tests. Locally python -m pytest tests/system/cli/test_utils.py is 4 passed, and RED→GREEN holds (reverting the source fix fails both new tests). A re-run of the sr.ht jobs should clear the AUR flake.

[WhyNotHugo]

Please rebase on current main

[chiliec]

Rebased on current main — CHANGELOG conflict resolved (kept both the Python 3.14 and locked-db entries), code merged clean. Ready for another look.

[chiliec]

Rebased on current main (the branch was already on top of ceee70d, so this is a no-op rebase). Also fixed the tests-pypi failure: ruff BLE001 flagged the blind except BaseException in the two new tests — they now catch sqlite3.OperationalError explicitly, which is the only thing they raise. ruff check + ruff format --diff clean, and the three handle_cli_error tests pass locally.