Derive catalog song URLs from source IDs; Add OSS repo templates; Add…

[Jamesllllllllll]

Summary

Adds production-safe Sentry scaffolding, open-source community templates/docs, and catalog URL/schema cleanup.

What changed

• scaffolded Sentry for the Cloudflare app and backend workers
• kept Sentry opt-in via SENTRY_DSN, with production secrets managed in Cloudflare
• added issue templates, PR template, CODE_OF_CONDUCT.md, and CHANGELOG.md
• simplified catalog song download URLs to derive from source_song_id
• removed catalog_songs.source_url with a new D1 migration
• updated schema-version enforcement to track the latest checked-in migration only
• moved remote D1 migrations into production CI
• blocked remote-affecting npm scripts outside CI with clear local-development guidance
• updated README and deployment docs for local vs production migration workflow

How to test

• run npm run db:bootstrap:local
• run npm run lint
• run npm run typecheck
• run npm run test
• run npm run build
• verify the dashboard playlist still loads and candidate download links resolve correctly
• verify npm run db:migrate:remote is blocked locally with the helper message

Screenshots / UI notes

No major UI changes. Dashboard playlist behavior was preserved while catalog source URLs became derived instead of stored.

Checklist

• Local D1 migrations apply cleanly
• Sample seed matches the updated schema
• Lint passes
• Typecheck passes

[github-actions]

Preview deployment updated.

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