feat: Phase 3 - per-peer internet routing via exit cell

Adds the ability to route a specific peer's internet traffic through a
connected cell acting as an exit relay.

Cell A side:
- PUT /api/peers/<peer>/route-via {"via_cell": "cellB"} sets route_via
- Updates WG AllowedIPs to include 0.0.0.0/0 for the exit cell peer
- Adds ip rule + ip route in policy table inside cell-wireguard so the
  specific peer's traffic egresses via cellB's WG IP
- Sets exit_relay_active on the cell link and pushes use_as_exit_relay=True
  to cellB via peer-sync

Cell B side:
- Receives use_as_exit_relay in the peer-sync payload
- Calls apply_cell_rules(..., exit_relay=True) to add FORWARD -o eth0 ACCEPT
- Stores remote_exit_relay_active flag for startup recovery

Startup recovery:
- apply_all_cell_rules passes exit_relay=remote_exit_relay_active (cellB)
- _apply_startup_enforcement reapplies ip rule for each peer with route_via (cellA)
  since policy routing rules don't survive container restart

peer_registry gets route_via field with lazy migration.
22 new tests across test_cell_link_manager, test_peer_registry, test_peer_route_via.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

api/routes/peers.py

@@ -183,6 +183,77 @@ def update_peer(peer_name):
         return jsonify({"error": str(e)}), 500
 
 
+@bp.route('/api/peers/<peer_name>/route-via', methods=['PUT'])
+def set_peer_route_via(peer_name):
+    """Route a peer's internet traffic through a connected exit cell.
+
+    Body: {"via_cell": "cellB"} to enable, {"via_cell": null} to disable.
+    On enable: updates WG AllowedIPs and adds policy routing rule inside
+    cell-wireguard so the peer's packets egress through the exit cell.
+    On disable: reverts AllowedIPs and removes the ip rule.
+    Also signals the exit cell to add/remove the FORWARD-to-eth0 firewall rule.
+    """
+    try:
+        from app import peer_registry, wireguard_manager, cell_link_manager
+        data = request.get_json(silent=True)
+        if data is None or 'via_cell' not in data:
+            return jsonify({'error': 'via_cell field required (string or null)'}), 400
+
+        via_cell = data['via_cell']
+        if via_cell is not None and not isinstance(via_cell, str):
+            return jsonify({'error': 'via_cell must be a string or null'}), 400
+
+        peer = peer_registry.get_peer(peer_name)
+        if not peer:
+            return jsonify({'error': 'Peer not found'}), 404
+
+        peer_ip = peer.get('ip', '').split('/')[0]
+        if not peer_ip:
+            return jsonify({'error': 'Peer has no IP assigned'}), 400
+
+        old_via = peer.get('route_via')
+
+        # Remove old routing if switching away from a previous exit cell
+        if old_via and old_via != via_cell:
+            old_link = next(
+                (l for l in cell_link_manager.list_connections()
+                 if l['cell_name'] == old_via), None
+            )
+            if old_link:
+                wireguard_manager.update_cell_peer_allowed_ips(
+                    old_link['public_key'], old_link['vpn_subnet'],
+                    add_default_route=False)
+            wireguard_manager.remove_peer_route_via(peer_ip)
+            try:
+                cell_link_manager.set_exit_relay_active(old_via, False)
+            except Exception as e:
+                logger.warning(f"set_exit_relay_active(False) for {old_via!r} failed: {e}")
+
+        # Apply new routing
+        if via_cell:
+            link = next(
+                (l for l in cell_link_manager.list_connections()
+                 if l['cell_name'] == via_cell), None
+            )
+            if not link:
+                return jsonify({'error': f"Cell {via_cell!r} not connected"}), 404
+            wireguard_manager.update_cell_peer_allowed_ips(
+                link['public_key'], link['vpn_subnet'], add_default_route=True)
+            wireguard_manager.apply_peer_route_via(peer_ip, via_wg_ip=link['dns_ip'])
+            try:
+                cell_link_manager.set_exit_relay_active(via_cell, True)
+            except Exception as e:
+                logger.warning(f"set_exit_relay_active(True) for {via_cell!r} failed: {e}")
+
+        updated_peer = peer_registry.set_route_via(peer_name, via_cell)
+        return jsonify({'message': f"Route-via for '{peer_name}' updated", 'peer': updated_peer})
+    except ValueError as e:
+        return jsonify({'error': str(e)}), 404
+    except Exception as e:
+        logger.error(f"Error setting route-via for {peer_name}: {e}")
+        return jsonify({'error': str(e)}), 500
+
+
 @bp.route('/api/peers/<peer_name>/clear-reinstall', methods=['POST'])
 def clear_peer_reinstall(peer_name):
     try: