feat(federation): scaffold peer-activation recovery module

Empty stubs for onPeerActivated, resetOutboxBackoff, syncPeerMutationLog,
and startupBootstrapSync. Functions are filled in by subsequent tasks
following TDD cycles.

packages/server/src/utils/federationPeerActivation.ts

@@ -0,0 +1,81 @@
+import { getDb } from '../db/index.js';
+import * as schema from '../db/schema.js';
+import { and, eq } from 'drizzle-orm';
+import { isFederationRelayEnabled } from './federationOutbox.js';
+import { buildFederationHeaders, getOurOrigin } from './federationAuth.js';
+import type { FederationRelayEvent } from '@backspace/shared';
+
+export type PeerActivationReason =
+  | 'initiate_accepted'
+  | 'accept_rejected_override'
+  | 'accept_awaiting_approval'
+  | 'accept_pending'
+  | 'accept_new'
+  | 'approval_handshake'
+  | 'health_check_recovery'
+  | 'ensure_peered'
+  | 'startup_bootstrap';
+
+// Dedup: concurrent activations for the same peerId share one promise.
+const inFlightActivation = new Map<string, Promise<void>>();
+
+/**
+ * Called whenever federation_peers.status transitions to 'active' for any reason.
+ * Two independent invariants — both run unconditionally:
+ *   1. Reset outbox backoff (nextRetryAt = now, attempts = 0) for this peer.
+ *   2. Pull-sync mutation log from peer's /api/federation/sync since lastSyncedAt.
+ *
+ * Call sites (must remain exhaustive — grep `onPeerActivated(` to audit):
+ *   - routes/federation.ts /peer/initiate activation
+ *   - routes/federation.ts /peer/accept existing-rejected override
+ *   - routes/federation.ts /peer/accept existing-awaiting_approval
+ *   - routes/federation.ts /peer/accept existing-pending
+ *   - routes/federation.ts /peer/accept new-peer
+ *   - routes/federation.ts /approval-requests/:id/approve
+ *   - utils/federationWorker.ts health check recovery
+ *   - utils/federationPeering.ts ensurePeered/performHandshake
+ *   - utils/federationWorker.ts startup bootstrap (via startupBootstrapSync)
+ *
+ * Deduplicated by peerId — concurrent calls share one promise.
+ */
+export async function onPeerActivated(
+  peerId: string,
+  reason: PeerActivationReason,
+): Promise<void> {
+  // Stub — implemented in Task 4.
+  void peerId;
+  void reason;
+}
+
+/**
+ * Reset all outbox backoff state for a peer (nextRetryAt = now, attempts = 0).
+ * Unconditional across all entries of the peer — see spec §Invariant 1.
+ */
+export function resetOutboxBackoff(peerId: string): void {
+  // Stub — implemented in Task 2.
+  void peerId;
+}
+
+/**
+ * Pull-sync mutation log from the peer's /api/federation/sync endpoint.
+ * Runs three contextType passes (dm, friend, profile), paginating each.
+ * Updates peer.lastSyncedAt to Date.now() on success; leaves it untouched
+ * on transient failure so the next activation retries.
+ */
+export async function syncPeerMutationLog(
+  peerId: string,
+  reason: PeerActivationReason,
+): Promise<void> {
+  // Stub — implemented in Task 3.
+  void peerId;
+  void reason;
+}
+
+/**
+ * Startup bootstrap — scan for freshly-peered rows (status='active', lastSyncedAt=0)
+ * and run onPeerActivated for each. Replaces runInitialSyncForNewPeers.
+ * Invoked from startFederationWorkers.
+ */
+export async function startupBootstrapSync(): Promise<void> {
+  // Stub — implemented in Task 5.
+}