Add configurable AI completion audio cues

[paullizer]

Summary

• add an admin-controlled, opt-in AI response completion audio capability
• provide ten locally bundled cues with profile preview, mute, sound, and volume controls
• deliver exactly-once background cues through live stream events plus server-authoritative notification polling
• synchronize preferences across tabs and suppress historical, foreground, disabled-period, failed, and duplicate events

Validation

• python functional_tests/test_chat_completion_audio_cues.py
• required route policy inventory, unauthenticated contract, and coverage tests
• affected background unread, notification polling, and user settings cache regressions
• JavaScript syntax checks, Python compilation, and git diff --check
• focused Playwright test added; local execution skipped because no authenticated UI environment was configured

Closes #1062