Add multi-camera investigation workspace

[matteius]

Summary

• persist immutable capture-time camera UUIDs on recordings and detections, with safe legacy backfill
• add a camera-authorized multi-track investigation timeline endpoint
• add an experimental synchronized investigation workspace with explicit gaps, shared UTC cursor, playback-speed controls, skip-common-gap mode, and primary-audio selection
• link selected recordings and the existing timeline into investigation windows
• enforce capture-identity replay authorization for recording metadata, playback, download, and thumbnails

Validation

• cmake --build build -j2
• ASAN_OPTIONS=detect_leaks=0 build/bin/test_api_handlers_investigations
• ASAN_OPTIONS=detect_leaks=0 build/bin/test_db_detections
• ASAN_OPTIONS=detect_leaks=0 build/bin/test_db_recordings_extended
• ASAN_OPTIONS=detect_leaks=0 build/bin/test_authorization
• ASAN_OPTIONS=detect_leaks=0 build/bin/test_httpd_utils
• npm test -- --runInBand (17 suites, 112 tests)
• npm run build
• git diff --check