Catch all Historian sink exceptions in EventEmitter.emit (#90)

[randileeharper]

Summary

Fixes #90.

EventEmitter.emit only caught HistorianDeliveryError, but HttpHistorianSink._request re-raises other exceptions — httpx.HTTPStatusError on a 4xx response, ValueError on malformed JSON, httpx.RequestError after exhausting retries — which then propagated out of emit and failed the surrounding user-facing music operation. This violated the design rule that Historian delivery is best-effort.

Change

Broadened emit's except so the best-effort contract is a hard boundary:

• HistorianDeliveryError → logged at warning (unchanged behavior)
• any other Exception → caught, logged at error with a traceback (exc_info=True), and never propagated

This keeps genuine sink bugs visible in logs (the visibility concern from issue #47 that originally narrowed the catch to HistorianDeliveryError) while ensuring a Historian outage or bug can never break a pause() / play() / etc. call.

Also hoisted the per-call import logging to a module-level _log logger.

Test changes

• Updated the issue Broad except Exception handlers hide unexpected failures #47 regression test (test_unexpected_sink_error_propagates_instead_of_being_swallowed → test_unexpected_sink_error_is_logged_and_does_not_break_operations): it now asserts the operation succeeds and the RuntimeError is logged at error level with a traceback, instead of asserting it propagates.
• Added test_http_status_error_from_sink_does_not_break_operations: covers the specific httpx.HTTPStatusError (4xx) scenario from issue Non-delivery exceptions from Historian sink can break operations #90 flowing through EventEmitter.emit.

Verification

Exact commands run (per AGENTS.md, using the project virtualenv):

.venv/bin/python -m pytest -q                          # 252 passed
.venv/bin/python -m pytest tests/test_historian.py -q  # 18 passed
.venv/bin/python -m ruff check vesper tests            # All checks passed!
.venv/bin/python -m mypy vesper                        # Success: no issues found in 37 source files
.venv/bin/python -m compileall -q vesper tests

Closes #90.