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>

api/wireguard_manager.py

@@ -183,13 +183,11 @@ class WireGuardManager(BaseServiceManager):
             f'PostUp = iptables -A FORWARD -i %i -j DROP; '
             f'iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE; '
             f'{hairpin}'
-            f'{dnat_up}; '
-            f'sysctl -q net.ipv4.conf.all.rp_filter=0 || true\n'
+            f'{dnat_up}\n'
             f'PostDown = iptables -D FORWARD -i %i -j DROP 2>/dev/null || true; '
             f'iptables -t nat -D POSTROUTING -o eth0 -j MASQUERADE 2>/dev/null || true; '
             f'{hairpin_down}'
-            f'{dnat_down}; '
-            f'sysctl -q net.ipv4.conf.all.rp_filter=1 || true\n'
+            f'{dnat_down}\n'
         )
 
     @staticmethod