neptun: add Headscale + Headplane

Headscale is the tailnet control server every host's services/tailscale.nix
already points at (--login-server=https://vpn.mgaction.town). MagicDNS
base_domain "hosts.mgaction.town" matches the "jupiter.hosts.mgaction.town"
names already used in this repo's Caddy vhosts.

Headplane is its web UI, running as headscale's own user (native process
integration, no container). No OIDC wired up - log in with a headscale API
key generated on the box. Both proxied through Caddy; headscale's vhost
needs flush_interval -1 since its node-update endpoint is a long-poll.

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

secrets/neptun.yaml

@@ -2,6 +2,7 @@ tailscale_authkey: ENC[AES256_GCM,data:CBMidfdilBn1HP3TAu9TCOpuwPnzVQ/3OGCsf0vhF
 darman_password: ENC[AES256_GCM,data:otLevSn6bXBrjCu7Dv9PN7Umwrb7CD1PhC1i+J8rp2mURMYVuwF/ZnrlPC7/RDZgINIRBOXOkG9lZo3UjuRyqrtvfUzIm1jGL51l1zvZ/07oNW6OUp4IZEQpoOmz1GuXBlx+yKRQeRDHYA==,iv:pnwp3uAsBoiJdF+QV27h0jztgFJKrIqFlmMT7JkcHoA=,tag:Zb9ziCGsLQt+TcJRCv8tWA==,type:str]
 zitadel_master_key: ENC[AES256_GCM,data:LDZwbpwZdQ03O8I2tvIMYnK3MoQTgvOYwrBt2UuCHbk=,iv:sM66sj3JO+bb0Sk2D7M/Ehk7t7VZneuvTRP4IkT1se0=,tag:YKQ4shwZ4sIDWdNehTugew==,type:str]
 zitadel_admin_password: ENC[AES256_GCM,data:q+Od3Juzvl/dVPkm0w1C42pjK7kU8ur8V5fl2JAdQkA=,iv:n8NFzQczJVeqe1ybBldbkv4QGj0xyf9/P+uvsHs3NjI=,tag:rkXvmC8kZZOrWRQd+mIanA==,type:str]
+headplane_cookie_secret: ENC[AES256_GCM,data:oXYRG4z16u6HS7zXoWrV2q/HL2o24n4UwVXnQvqBmbY=,iv:itAiy/w6ue4VzqO5xYnvSYN3uCLLmu52dvAAxZ2pCGc=,tag:MEFP171GpSQpAqUFsWY+VA==,type:str]
 sops:
     age:
         - enc: |
@@ -22,7 +23,7 @@ sops:
             Wptkf76aP9UpjhgNkxzedRebQPB7ti+UiVqCvLVimtuHcsm/NJPcRg==
             -----END AGE ENCRYPTED FILE-----
           recipient: age1hp72xyx2cnd05937e4eww95g5kdtn0wsf9j2nypw330pa69gfdxqn0lpkp
-    lastmodified: "2026-07-19T20:28:23Z"
-    mac: ENC[AES256_GCM,data:3dRmmSJqaKi8w1mtmQIaL8AynSUCH4WRUpUax5g+3qKrhQeect7TdCFyTU991mfmUnxfQ8nI7nyAI2DdAb/TTHxAjDgXd0Qn31olGYZRQFRV25YOiXozvXoHVjroW86IQDgh6UBp5BkpU+HrfCfLs/opAx97jLu14M4sLLuu8R4=,iv:qmKCCX70IJ/JQvpjMW1eYM2cgjVx+1PS44ahhTXiOp0=,tag:hi7V9zwo4uZxAjEm3dUxBw==,type:str]
+    lastmodified: "2026-07-19T20:35:48Z"
+    mac: ENC[AES256_GCM,data:4cQsixlVeKbZmtnrBXuTb9H62QCyl1rn3hba7fh2EHO7jVJu6H5w93q0jZVkMSCz/iCqhQVjAg7pY1st/VpVKWjtiV2ys8N7Qpom4PzKBas+c8/5d1I84DYxR9yIPHYZPpEsFhhzjFh2V7g7gTR8yVHO7Wi2NG8R9Nhrhbwqcj8=,iv:Vr19xtQ/tQQoKVx7aXi/f9pl3mj5liwXCK/J/IFdx68=,tag:gCL1ydSdJsonAaaBXf+hCw==,type:str]
     unencrypted_suffix: _unencrypted
     version: 3.13.1