Encrypt, decrypt, and store backups locally

crypto.py and storage.py: the encrypt-and-write half of the pipeline. The pipefail check here is the highest-risk non-pure code in the project.

• crypto: encrypt(stdin, recipients, out) chains Popen so dump stdout flows into age -r … and on to the file, and checks both return codes. A pg_dump failure mid-stream must fail the backup even though age exits 0. This is the set -o pipefail equivalent, and getting it wrong is how you end up with truncated "successful" backups. decrypt(in, identity) -> stream. Recipients are public keys; the identity is supplied only at restore and never stored.
• storage: write(app, name, stream) writes atomically (temp then rename); list(app) -> [meta] returns name, size, mtime, and parsed time, newest first; open(app, name) -> stream.

Acceptance

• a mid-stream dump failure fails the backup despite age exiting 0, tested with a fake command that exits non-zero mid-stream (no Postgres needed).
• the output is a valid age file; decrypt with the matching identity reproduces the exact dump bytes.
• local writes are atomic, so a crashed run never leaves a half-file that looks valid; list returns sizes and parsed times.