Add AI agent onboarding and project context documentation

[Copilot]

No structured project overview existed that could be fed to an AI agent to bootstrap context for feature discussions and development conversations.

Changes

• PROJECT_CONTEXT.md (new, repo root) — comprehensive reference document covering:
  • Project purpose, live URL, and repo URL
  • Full tech stack table (backend: Django/DRF/PostgreSQL/NumPy/scikit-learn; frontend: React 19/TypeScript/Vite/Axios/YouTube IFrame API; infra: Docker/Nginx/Certbot)
  • Architecture diagram (Browser → Nginx → Gunicorn → Django → Postgres)
  • Data model field-level breakdown (Track, Artist, Album, GenreDortmund, GenreRosamerica)
  • All 16 audio features (11 perceptual + 5 MIREX moods) with normalization notes
  • Step-by-step recommendation algorithm (pre-filter → weight → cosine similarity → popularity re-rank)
  • Full REST API endpoint table with request/response shapes for /api/v1/recommend/
  • AcousticBrainz dataset details and ingest pipeline summary
  • Frontend component layout and auto-advance behavior
  • Deployment setup, required env vars, known limitations, and codebase conventions

[RadValentin]

I made some tweaks to the documents and I'm requesting the following changes.