Test quak backup end to end against a fake Ente server (closes #186)

test/cli/backup-end-to-end.test.ts runs backupCommand through run, as
bin/quak.ts does, with the real loadSession reading a session saved after
logging in, and every request answered by a fake of the Ente HTTP API serving
data encrypted with libsodium. It covers the backup tree of an account with
two albums of its own and one shared with it, a second run that downloads
nothing, a download that succeeds after two 503s, a 404 recorded in
failures.json with exit 1, a refresh answered 401 with exit 3, and --verify
putting back an original changed on disk.

The fake server and the account it logs in moved from the backup-metadata
test into test/ente-server.ts, unchanged, so both tests use them.

Model: opus-5-5

README.md

@@ -850,7 +850,8 @@ The library's primary surface is the cache-backed `Library`; the lower-level
 `Client` sits underneath it and is covered by the Design sections above. The
 test suite is the canonical, executable documentation — `test/library/` and
 `test/client/usage.test.ts` walk most operations, `test/cli/backup.test.ts`
-walks `lib.backup()`, and `yarn test` verifies them.
+walks `lib.backup()`, `test/cli/backup-end-to-end.test.ts` runs `quak backup`
+against a fake of the Ente HTTP API, and `yarn test` verifies them.
 
 ### Opening a library