fix: complete cross-cell peer-sync push (domain SNI + source-preserving NAT)

Finishes the transport repair (L1+L2 landed in 714fb9b). The push now works
end-to-end between linked cells — verified live: offer/permission state
propagates automatically and the cell_relay derives/reverts without manual steps.

L3 — push by domain, not bare IP (cell_link_manager): the push targeted
https://<vpn-ip>, but in DDNS/ACME mode Caddy only holds a cert for the cell's
domain, so the TLS handshake failed by IP. Target https://<remote-domain> with
`curl --resolve <domain>:443:<dns_ip>` — connect to the VPN IP over the tunnel
but present the domain as SNI/Host. remote_api_url is now domain-based; legacy
http://ip:3000 and https://ip URLs migrate on load.

L4 — preserve the real source for auth (firewall_manager): the blanket
`-o eth0 MASQUERADE` rewrote the push source, so the remote's X-Forwarded-For
source-subnet auth couldn't match. apply_cell_rules adds a tightly-scoped nat
POSTROUTING RETURN (linked-subnet → caddy:443 only) above the masquerade; the
host route returns Caddy's reply through the tunnel. Reviewed by pic-security:
WireGuard per-cell AllowedIPs + Caddy last-XFF (no trusted_proxies) keep this
un-spoofable; the API stays 127.0.0.1-only.

Also:
- validate remote-invite domain/dns_ip/endpoint/subnet at ingest (they reach a
  curl --resolve argv — block leading-dash argument-injection).
- remove the host subnet route on cell unlink (remove_cell_subnet_route); the
  route was never cleaned, leaving a stale subnet that made is_local_request
  treat it as local. Mock firewall side-effects in the affected unit tests.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

api/firewall_manager.py

@@ -438,11 +438,26 @@ def apply_cell_rules(cell_name: str, vpn_subnet: str, inbound_services: List[str
             _iptables(['-I', 'FORWARD', '-s', vpn_subnet, '-d', caddy_ip,
                        '-p', 'tcp', '--dport', '443',
                        '-m', 'comment', '--comment', tag, '-j', 'ACCEPT'])
+            # Preserve the linked cell's real VPN source on peer-sync traffic:
+            # the blanket `-o eth0 MASQUERADE` would rewrite it to cell-wireguard's
+            # bridge IP, and the remote side authenticates the push by matching the
+            # source (via X-Forwarded-For) to the cell's VPN subnet. RETURN before
+            # the MASQUERADE (inserted at the top of nat POSTROUTING). Caddy's reply
+            # to the real VPN IP routes back via the cell-subnet host route
+            # (ensure_cell_subnet_routes). The :80 service path keeps masquerade.
+            _iptables(['-t', 'nat', '-I', 'POSTROUTING', '-s', vpn_subnet,
+                       '-d', caddy_ip, '-p', 'tcp', '--dport', '443',
+                       '-m', 'comment', '--comment', tag, '-j', 'RETURN'])
 
         # Ensure reply traffic (e.g. ICMP, TCP ACKs) for connections initiated
         # by local peers to this cell is not dropped by the cell's catch-all DROP.
         ensure_forward_stateful()
 
+        # Host route so Caddy's peer-sync reply (to the linked cell's un-masqueraded
+        # VPN IP) leaves via cell-wireguard rather than the default gateway. Added at
+        # startup for all links; ensure it on runtime link-add too. Idempotent.
+        ensure_cell_subnet_routes([{'vpn_subnet': vpn_subnet}])
+
         logger.info(
             f"Applied cell rules for {cell_name} ({vpn_subnet}): "
             f"inbound={inbound_services} exit_relay={exit_relay}"
@@ -689,6 +704,25 @@ def ensure_cell_subnet_routes(cell_links: List[Dict[str, Any]]) -> None:
             logger.warning(f'ensure_cell_subnet_routes: {subnet}: {e}')
 
 
+def remove_cell_subnet_route(vpn_subnet: str) -> None:
+    """Remove the host route for a disconnected cell's VPN subnet (idempotent).
+
+    Counterpart to ensure_cell_subnet_routes. Without it the route lingers after a
+    cell is unlinked — blackholing that subnet via cell-wireguard, and (on a host
+    that runs the API/tests directly, e.g. a dev box) making is_local_request /
+    _local_subnets treat the stale subnet as locally attached.
+    """
+    if not vpn_subnet:
+        return
+    WG_BRIDGE_IP = '172.20.0.9'
+    try:
+        _run(['docker', 'run', '--rm', '--network', 'host', '--cap-add', 'NET_ADMIN',
+              'alpine', 'ip', 'route', 'del', vpn_subnet, 'via', WG_BRIDGE_IP],
+             check=False)
+    except Exception as e:
+        logger.warning(f'remove_cell_subnet_route: {vpn_subnet}: {e}')
+
+
 # ---------------------------------------------------------------------------
 # DNS ACL (CoreDNS Corefile generation)
 # ---------------------------------------------------------------------------