F2.1a: record the backend walking-skeleton ADR

[Flow-Fly]

Summary

• record the owner-approved backend walking-skeleton boundaries as ADR 0001
• keep the existing local-first client at the root and define shared/ plus server/ workspace boundaries
• select Node 22, TypeScript, Hono, portable PostgreSQL/Drizzle, and a dedicated S3-compatible blob port
• preserve the current Object Storage frontend while targeting api.pixel-forge.app on Scaleway Serverless Containers
• gate Serverless SQL behind compatibility and public-network security evidence, with Managed PostgreSQL as the fallback
• keep Cine and every cloud mutation outside this PR

Closes #270.

Decision boundaries

This PR changes documentation only. It creates no workspace, dependency, server, database, bucket, secret, DNS record, migration, or deployment.

#266 and later backend implementation issues remain blocked until this PR is reviewed and merged.

Verification

• npm run test:run — 91 files, 504 tests passed
• npm run lint — no errors; five existing warnings
• npm run build
• npm run fallow:audit — no findings in the changed file
• npx prettier --check specs/adr/0001-backend-walking-skeleton.md
• git diff --check

Review focus

• package dependency direction
• exact ownership of the portable ProjectFile contract
• Serverless SQL evidence/fallback gate
• separation of public static assets and private project blobs
• operational approval and rollback boundaries