fix: remove sysctl rp_filter from WireGuard PostUp/PostDown

sysctl writes to /proc/sys/net/ are blocked in unprivileged containers
(NET_ADMIN only, no SYS_ADMIN). The rp_filter=0 call at the end of
PostUp caused wg-quick to tear down wg0 immediately on every start,
putting cell-wireguard into a crash loop.

Remove the sysctl lines from both the seed (setup_cell.py) and the
API-regenerated (wireguard_manager.py) wg0.conf. Reverse-path filtering
is an optimisation, not required for VPN functionality; the iptables
FORWARD/MASQUERADE/DNAT rules all still work correctly without it.

Found during clean-install hardening verification on pic1 (f4b8d5c).

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

scripts/setup_cell.py

@@ -147,11 +147,9 @@ def write_wg0_conf(private_key: str, address: str, port: int):
         f'Address = {address}\n'
         f'ListenPort = {port}\n'
         f'PostUp = iptables -A FORWARD -i %i -j ACCEPT; '
-        f'iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE; '
-        f'sysctl -q net.ipv4.conf.all.rp_filter=0\n'
+        f'iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE\n'
         f'PostDown = iptables -D FORWARD -i %i -j ACCEPT; '
-        f'iptables -t nat -D POSTROUTING -o eth0 -j MASQUERADE; '
-        f'sysctl -q net.ipv4.conf.all.rp_filter=1\n'
+        f'iptables -t nat -D POSTROUTING -o eth0 -j MASQUERADE\n'
     )
     with open(wg_conf, 'w') as f:
         f.write(content)