refactor: Extract key detection into shared module to eliminate duplication

Commit notes

- Create new shared module: backend/src/audio/key-detection.ts - Contains all key detection logic, constants, and profiles - Exports: detectMusicalKey, extractKeyFromFilename, getCamelotCode - Exports: KEY_NAMES, CAMELOT_MAJOR, CAMELOT_MINOR, KEY_PROFILES, etc. - Update audio-analysis.ts to import from shared module (~1000 lines removed) - Update audio-analysis-worker.ts to import from shared module (~250 lines removed) - Both files now use the same key detection implementation - All tests pass

Co-authored-by: armin.naimi <[email redacted]>