fix: use a portable file lock for telemetry installation ids

[Frankie-Xu]

Summary

• fcntl.flock is POSIX-only, so importing shared.services.telemetry crashed on Windows before an installation id was generated (ADR-0004 stable anonymous id).
• Keep fcntl.flock on Unix. On Windows use msvcrt.locking plus an in-process mutex (no new dependency).
• This unblocks collecting/running API contract tests on Windows, including the HTTP cases in fix: deduplicate concurrent upload parse dispatch #351.

Test plan

• make lint
• uv run pytest packages/shared-python/shared/tests/test_telemetry_identity.py -q — 7 passed (explicit id, generate UUID, concurrent threads, Windows import without fcntl)
• uv run pytest apps/api/tests/contract/test_self_hosted_telemetry_contract.py -k installation_id -q — 3 passed
• uv run pytest apps/api/tests/contract/test_s3_event_contract.py --collect-only -q — 32 tests collected (previously failed on import fcntl)
• pyright on identity.py is clean on Windows. Pre-existing PipeConnection errors in ocr_pages.py are unchanged and out of scope.

Made with Cursor

[Frankie-Xu]

Please approve Actions workflows for this first-time fork PR so CI can run.

[Frankie-Xu]

Rebased onto current main (9489fa2c, includes #414/#415). No further code changes in this rebase; still waiting on first-time fork Actions workflow approval.