Onboard repository to Copilot cloud agent

[Copilot]

Adds the two files needed to make the Copilot cloud agent productive on first contact with this repo.

.github/copilot-instructions.md

Comprehensive reference the agent reads before every session:

• Architecture — Django 5.2 API + React 19 SPA structure, FeatureStore singleton (features_and_index.npz not in git, mocked in tests), PostgreSQL pg_trgm requirement, URL/serializer conventions
• Environment variables — table of which vars are required for tests vs production (YOUTUBE_API_KEY is exempt during manage.py test)
• Commands — backend (coverage run manage.py test, min 80%), frontend (npm run build/lint), migrations
• Testing conventions — APITestCase + factory_boy, mock targets (rec.recommend, FeatureStore), reset_sequence in tearDownClass
• Endpoint addition checklist — model → serializer → view → url → test → @extend_schema
• Common errors & workarounds — missing .npz, missing pg_trgm extension, FeatureStore reinitialization error, missing YOUTUBE_API_KEY

.github/workflows/copilot-setup-steps.yml

Pre-warms the agent's environment so it can run tests immediately without trial-and-error dependency discovery:

• postgres:17 service container with health check
• Python 3.13.13 + requirements-test.txt (pip-cached)
• CREATE EXTENSION IF NOT EXISTS pg_trgm via psql
• python manage.py migrate
• Node 24 + npm ci in frontend/ (npm-cached)

[RadValentin]

Made changes so the agent environment has:

• YT API key injected through repository secrets
• A cached version of the AcousticBrainz sample dataset (100k entries) that it can run the ingest pipeline on
• Actions running on Node 24

Up next: I need to personally review the copilot-instructions.md and ensure they're in line with how the project's supposed to work.