fix(federation): close re-attach final-review findings — client/server domain normalization, merge attachment repoint, empty-domain guard, test hardening

docs/systems/api.md

@@ -14,7 +14,7 @@ POST /auth/login             { username, password } → { token, user }
 POST /auth/attach-proof      (JWT, rate-limited 5/15min)  { targetDomain } → { token }   (AttachProofResponse)
 ```
 
-**`POST /auth/attach-proof`** — JWT-authenticated. Mints a one-time 256-bit token (`randomBytes(32).toString('hex')`) for the logged-in **native** user, stored in `federation_attach_proofs` bound to `{ homeUserId, targetDomain, expiresAt = now+60s }`; expired/used rows are janitored on each mint. The token is handed to the peer named by `targetDomain`, which redeems it via `POST /federation/verify-attach-proof` to re-attach the caller's detached account there. See `federation.md` "S2S Detached-Account Re-Attach Proof" and re-attach spec §3.1.
+**`POST /auth/attach-proof`** — JWT-authenticated. Mints a one-time 256-bit token (`randomBytes(32).toString('hex')`) for the logged-in **native** user, stored in `federation_attach_proofs` bound to `{ homeUserId, targetDomain, expiresAt = now+60s }`; expired rows are janitored on each mint (the delete targets `expires_at < now`; a used-but-unexpired row lingers until it expires). The token is handed to the peer named by `targetDomain`, which redeems it via `POST /federation/verify-attach-proof` to re-attach the caller's detached account there. See `federation.md` "S2S Detached-Account Re-Attach Proof" and re-attach spec §3.1.
 
 **`POST /auth/login`** — request/response shape unchanged, but two internal controls from instance-epoch self-healing gate the flow: (1) an account with `federationHomeOrphaned = 1` (home instance factory-reset) is **detached** — a sovereign local account whose local password hash is the sole authority; it logs in normally with that local password (the detach pivot removed the old pre-verification freeze), and the flag's only login effect is to permanently disable self-heal (step 7); (2) for non-detached federated accounts, the password self-heal runs an **epoch guard** — it re-hashes the stale local password only if the home instance's authenticated epoch (`fetchPeerEpoch`) matches the trusted baseline, failing closed when the epoch differs or can't be determined. A detached account can be re-bound to the owner's new home identity via `POST /users/@me/reattach` (re-attach spec §3.2), which clears the flag and re-enables normal federated semantics. No wire-shape change. See `auth.md` §4.