feat(federation): peer_reset_pending guard during limbo window

docs/systems/social.md

@@ -293,6 +293,7 @@ As of 2026-04-25, the sender's home server owns the entire federated friend-add
 
 1. **Parse target.** If `body.username` contains no `@`, or the domain after `@` normalizes to this server's own host, fall through to the local-only path (unchanged).
 2. **resolveOriginFromHostname(targetDomain)** — resolves the target peer's full origin URL. Prefers a stored `federation_peers` row matching the typed host; falls back to mirroring `getOurOrigin()`'s scheme. Returns null → 400 `invalid_target_domain`.
+2a. **Limbo-window guard → 409 `peer_reset_pending`.** O(1) point lookup on the `federation_reset_events` origin PRIMARY KEY: if an **unresolved** row exists for `peerOrigin` (`origin = peerOrigin AND resolved_at IS NULL`), the peer was reset-detected (wipe-and-reinstall) but the admin has not yet re-peered — the local friendship/stub graph is still bound to the dead incarnation. Return 409 `peer_reset_pending` instead of the confusing `already_friends` (stale friendship) or `peer_rejected` (the `needs_attention` peer would otherwise trip `ensurePeered`). `peerOrigin` is the exact string `markPeerReset` journals (the peer's `federation_peers.origin`), so the match is a single indexed lookup; no reset in progress → one indexed miss → the normal path proceeds unchanged. See `docs/systems/federation.md` (instance-epoch self-healing) and the design spec §5.3. The equivalent guard runs on federated DM-create (`POST /api/dm`, `dm.ts`).
 3. **Authority defense.** If the calling user's `homeInstance` is set and does not normalize to this server's own host (checked via `normalizeOriginForCompare`), return 403 `not_authoritative_for_sender`. Prevents replicated/federated users from queueing relay events the home server isn't authoritative for. Runs before peering to fail fast.
 4. **ensurePeered(peerOrigin)** — blocks on the result. Status → HTTP mapping:
    - `'active'` → continue