#30: stop enrolment from killing the auth service, and from enabling MFA without recovery codes

Confirming a second factor returned "Upstream service is unreachable" - the
Next proxy's message for a backend it cannot reach. The backend was not
unreachable, it was dead: the process had just been OOM-killed.

generateRecoveryCodes hashed all ten codes with Promise.all. Argon2id here is
configured for 64 MiB per hash, so ten concurrently reserve 640 MiB against a
container limited to 512 MiB. Hashing is now sequential, peaking at 64 MiB and
taking a few seconds - the right trade for something that happens once per
enrolment. A timing-based test covers it, against the real hash rather than a
mock, because the cost being guarded is memory held concurrently and only the
real implementation reserves it.

The worse half is what the crash left behind. The enrolment row was marked
CONFIRMED before the codes were generated, so the account came out of this with
a second factor enabled, no recovery codes, and nobody having seen any: locked
behind an authenticator entry with no way back. That is how an account ends up
"assuming" it has a factor its owner cannot use.

The order is now verify, store the codes, then confirm. Anything that fails in
between leaves the enrolment pending and retryable, which is the recoverable
end of that failure.

Both bugs sat behind the one fixed in #28 - the code check failed first, so the
crash was never reached.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

services/auth/src/mfaHelpers.ts

@@ -10,7 +10,19 @@ export async function generateRecoveryCodes(count = 10): Promise<{ rawCodes: str
   for (let i = 0; i < count; i++) {
     rawCodes.push(crypto.randomBytes(6).toString("hex").substring(0, 10).toUpperCase());
   }
-  const hashedCodes = await Promise.all(rawCodes.map((c) => hashPassword(c)));
+  // Hashed one at a time, deliberately. Argon2id is configured for 64 MiB per
+  // hash, so ten of them in parallel reserve 640 MiB - more than the auth
+  // container is allowed - and the process was OOM-killed partway through
+  // enrolment. From the browser that surfaced as "upstream service is
+  // unreachable"; in the database it left an account with a confirmed second
+  // factor and no recovery codes, because the row is written before this runs.
+  //
+  // Sequentially it peaks at 64 MiB and takes a few seconds, which is the
+  // right trade for an operation that happens once per enrolment.
+  const hashedCodes: string[] = [];
+  for (const code of rawCodes) {
+    hashedCodes.push(await hashPassword(code));
+  }
   return { rawCodes, hashedCodes };
 }