feat(federation): add /users/by-home-id reverse lookup for stub backfill

HMAC-authenticated, rate-limited (60/min/peer) endpoint that resolves a
homeUserId on this instance to its canonical username + profile snapshot.
Native non-deleted users only. Mirrors /users/lookup's auth shape.

Adds lookupRemoteUserByHomeId to federationLookup.ts as the client-side
helper. Used by the upcoming stub-backfill worker on peers that hold legacy
snowflake-named replicas of users now visible by their real handle.

packages/server/src/routes/federation.ts

@@ -2336,6 +2336,94 @@ export async function federationRoutes(app: FastifyInstance): Promise<void> {
     },
   );
 
+  // ─── POST /api/federation/users/by-home-id ──────────────────────────────────
+  // Server-to-server: reverse-lookup a homeUserId to its canonical username +
+  // profile snapshot. Used by the stub-username backfill worker on peers that
+  // hold legacy snowflake-named replicas of users now visible by their real
+  // handle. Same auth+rate-limit shape as /users/lookup.
+  app.post<{ Body: { homeUserId?: unknown } }>(
+    '/api/federation/users/by-home-id',
+    { bodyLimit: 4 * 1024 },
+    async (request, reply) => {
+      const db = getDb();
+
+      // 1. Verify HMAC headers
+      const fedHeaders = parseFederationHeaders(request.headers as Record<string, string | string[] | undefined>);
+      if (!fedHeaders) {
+        return reply.code(401).send({ error: 'Missing or malformed federation headers', statusCode: 401 });
+      }
+
+      const peer = db
+        .select()
+        .from(schema.federationPeers)
+        .where(eq(schema.federationPeers.origin, fedHeaders.origin))
+        .get();
+
+      if (!peer || peer.status !== 'active') {
+        return reply.code(403).send({ error: 'Unknown or inactive peer', statusCode: 403 });
+      }
+
+      if (isLookupRateLimited(peer.origin)) {
+        return reply.code(429).header('Retry-After', '60').send({ error: 'Rate limit exceeded', statusCode: 429 });
+      }
+
+      const bodyString = JSON.stringify(request.body);
+      if (!verifyPeerSignature(bodyString, fedHeaders.signature, fedHeaders.timestamp, fedHeaders.nonce, peer)) {
+        return reply.code(401).send({ error: 'Invalid signature', statusCode: 401 });
+      }
+
+      // Replay protection
+      if (fedHeaders.nonce) {
+        if (isNonceDuplicate(peer.origin, fedHeaders.nonce)) {
+          return reply.code(409).send({ error: 'Duplicate nonce — possible replay', statusCode: 409 });
+        }
+      } else if (peer.nonceSupported) {
+        return reply.code(401).send({ error: 'Nonce required — peer previously supported nonces', statusCode: 401 });
+      }
+
+      // 2. Validate body
+      const rawId = (request.body as { homeUserId?: unknown } | null)?.homeUserId;
+      if (typeof rawId !== 'string' || rawId.trim().length === 0) {
+        return reply.code(400).send({ error: 'homeUserId is required (string)', statusCode: 400 });
+      }
+      const homeUserId = rawId.trim();
+
+      // 3. Native-only lookup. Match by id (canonical native id) OR home_user_id
+      // (backfilled column natives carry to satisfy tier-1 lookups). Excludes
+      // tombstoned and replicated stubs.
+      const user = db
+        .select()
+        .from(schema.users)
+        .where(
+          and(
+            eq(schema.users.isDeleted, 0),
+            isNull(schema.users.homeInstance),
+            or(eq(schema.users.id, homeUserId), eq(schema.users.homeUserId, homeUserId)),
+          ),
+        )
+        .get();
+
+      if (!user) {
+        return reply.code(200).send({ found: false });
+      }
+
+      return reply.code(200).send({
+        found: true,
+        user: {
+          homeUserId: user.homeUserId ?? user.id,
+          username: user.username,
+          profile: {
+            displayName: user.displayName,
+            avatar: user.avatar,
+            avatarColor: user.avatarColor,
+            banner: user.banner,
+            bio: user.bio,
+          },
+        },
+      });
+    },
+  );
+
   // ─── POST /api/federation/sync ──────────────────────────────────────────────
   // Server-to-server: checkpoint catch-up sync. A peer calls this after downtime
   // to retrieve missed DM mutations from the mutation log.

packages/server/src/utils/federationLookup.ts

@@ -75,3 +75,65 @@ export async function lookupRemoteUser(peerOrigin: string, username: string): Pr
     profile: json.user.profile,
   };
 }
+
+/**
+ * Reverse-lookup: ask the peer for a user by homeUserId. Used by the stub
+ * backfill worker to translate legacy snowflake-named stubs into realname-named
+ * stubs. Mirrors lookupRemoteUser's auth + error semantics.
+ *
+ * `not_found` here means "the peer does not host a native non-deleted user
+ * with that homeUserId" — including the tombstone case. Caller should leave
+ * the local stub untouched and retry on the next peer activation.
+ */
+export async function lookupRemoteUserByHomeId(peerOrigin: string, homeUserId: string): Promise<LookupResult> {
+  const db = getDb();
+  const peer = db
+    .select()
+    .from(schema.federationPeers)
+    .where(eq(schema.federationPeers.origin, peerOrigin))
+    .get();
+
+  if (!peer) {
+    throw new Error(`lookupRemoteUserByHomeId: no peer record for ${peerOrigin}`);
+  }
+
+  const body = JSON.stringify({ homeUserId });
+  const headers = buildFederationHeaders(body, peer.hmacSecret, getOurOrigin());
+
+  let response: Response;
+  try {
+    response = await fetch(`${peerOrigin}/api/federation/users/by-home-id`, {
+      method: 'POST',
+      headers,
+      body,
+      signal: AbortSignal.timeout(LOOKUP_TIMEOUT_MS),
+    });
+  } catch {
+    return { ok: false, reason: 'unreachable' };
+  }
+
+  if (response.status === 429) {
+    const raw = Number(response.headers.get('Retry-After') ?? '60');
+    const retryAfter = Number.isFinite(raw) ? raw : 60;
+    return { ok: false, reason: 'rate_limited', retryAfter };
+  }
+
+  if (!response.ok) {
+    throw new Error(`lookupRemoteUserByHomeId: peer ${peerOrigin} returned HTTP ${response.status}`);
+  }
+
+  const json = (await response.json()) as FederationUserLookupResponse;
+  if (!json) {
+    throw new Error(`lookupRemoteUserByHomeId: peer ${peerOrigin} returned empty body`);
+  }
+  if (json.found !== true || !json.user || typeof json.user.homeUserId !== 'string') {
+    return { ok: false, reason: 'not_found' };
+  }
+
+  return {
+    ok: true,
+    homeUserId: json.user.homeUserId,
+    username: json.user.username,
+    profile: json.user.profile,
+  };
+}