Architecture decision records

Each record describes one decision that shapes the codebase, the context that led to it, and its consequences. Add a new record instead of rewriting an accepted one; mark the old one as superseded.

Record	Status
0001. Docker first, behind a backend interface	Accepted
0002. Snapshots are workspace tarballs	Accepted
0003. Flat API keys	Accepted
0004. Tag-driven releases	Accepted