sops-encrypt TURN password + Authentik client secret; homelab render-vps-configs

The IONOS netbird VPS held two credentials in plaintext that were the last
holdouts from the homelab's sops+age secrets pattern:
  - /root/turn-pass.txt  (coturn long-term-credential password)
  - PKCEAuthorizationFlow.ProviderConfig.ClientSecret inline in
    /opt/management.json (Authentik OIDC client secret)

This commit moves both into sops-encrypted YAML in the repo and adds a render
command that recreates the VPS config files from templates + decrypted secrets:

* secrets/turn-shared-secret.yaml      — encrypted `password: <coturn pwd>`
* secrets/netbird-authentik-oidc.yaml  — encrypted `client_secret: <...>`
  Both recipients = hubris + apps + republic-laptop (same 3 as hello.yaml).
* vps/turnserver.conf.tmpl + vps/management.json.tmpl — templates with
  {{TURN_PASSWORD}} + {{AUTHENTIK_CLIENT_SECRET}} placeholders.
* bin/homelab new subcommand `render-vps-configs`:
  - Decrypts both secrets locally (works on any recipient).
  - Substitutes placeholders into templates.
  - Diffs against current VPS state via ssh, prompts, applies atomically
    (write `.new` then mv), restarts coturn + netbird-mgmt.
  - --dry-run + -y flags. Hops through hubris when not running on hubris
    itself, since VPS sshd is locked to hubris's pubkey.
* inventory.yaml adds the VPS as `kind: external` (new kind; reserved for
  ssh-managed hosts that aren't homelab clients themselves — no age key,
  no /opt/homelab-context). hosts/netbird-vps.yaml regenerated.
* SHARED_SECRETS list includes both new secrets so re-keys on enrollment
  changes pick them up automatically.

After this lands + the 5-min sync propagates to hubris, run from hubris PVE
shell (or any client; hubris just skips the extra ssh hop):

  homelab render-vps-configs --dry-run    # see plan, no changes
  homelab render-vps-configs -y           # apply + restart services

Once verified working, the plaintext `/root/turn-pass.txt` should be deleted
on the VPS (the rendered /etc/turnserver.conf no longer needs it as a
reference).

vps/management.json.tmpl

@@ -0,0 +1,75 @@
+{
+    "_comment": "Rendered from this template by `homelab render-vps-configs`. DO NOT edit /opt/management.json on the VPS directly; it is recreated from this template on each render. Secret placeholders are {{TURN_PASSWORD}} and {{AUTHENTIK_CLIENT_SECRET}}.",
+    "Stuns": [
+        {"Proto": "udp", "URI": "stun:stun.l.google.com:19302", "Username": "", "Password": null},
+        {"Proto": "udp", "URI": "stun:stun1.l.google.com:19302", "Username": "", "Password": null},
+        {"Proto": "udp", "URI": "stun:stun.cloudflare.com:3478", "Username": "", "Password": null}
+    ],
+    "TURNConfig": {
+        "Turns": [
+            {"Proto": "tcp", "URI": "turn:netbird.hubris.network:3478?transport=tcp", "Username": "netbird", "Password": "{{TURN_PASSWORD}}"}
+        ],
+        "CredentialsTTL": "12h",
+        "Secret": "not-used-when-time-based-false",
+        "TimeBasedCredentials": false
+    },
+    "Relay": {
+        "Addresses": ["rels://netbird.hubris.network:443"],
+        "CredentialsTTL": "24h",
+        "Secret": "f6vaBSTqv53Jl9Fr+zUkzJ6iIsKv0RoYt++hKRqq58Q"
+    },
+    "Signal": {
+        "Proto": "https",
+        "URI": "netbird.hubris.network:443",
+        "Username": "",
+        "Password": null
+    },
+    "ReverseProxy": {
+        "TrustedHTTPProxies": ["172.30.0.0/24"],
+        "TrustedHTTPProxiesCount": 0,
+        "TrustedPeers": ["0.0.0.0/0"]
+    },
+    "Datadir": "",
+    "DataStoreEncryptionKey": "U60qK19PEpe6LSocYs1OR+qeoE2rUq6tN+W20NAC+gs=",
+    "StoreConfig": {"Engine": "sqlite"},
+    "DisableDefaultPolicy": false,
+    "HttpConfig": {
+        "Address": "0.0.0.0:80",
+        "AuthIssuer": "https://auth.hubris.network/application/o/netbird/",
+        "AuthAudience": "netbird-dashboard",
+        "AuthUserIDClaim": "sub",
+        "AuthKeysLocation": "https://auth.hubris.network/application/o/netbird/jwks/",
+        "OIDCConfigEndpoint": "https://auth.hubris.network/application/o/netbird/.well-known/openid-configuration",
+        "IdpSignKeyRefreshEnabled": true,
+        "CertFile": "",
+        "CertKey": ""
+    },
+    "IdpManagerConfig": {
+        "ManagerType": "none"
+    },
+    "DeviceAuthorizationFlow": {
+        "Provider": "hosted",
+        "ProviderConfig": {
+            "ClientID": "netbird-dashboard",
+            "Audience": "netbird-dashboard",
+            "Domain": "auth.hubris.network",
+            "TokenEndpoint": "https://auth.hubris.network/application/o/token/",
+            "DeviceAuthEndpoint": "https://auth.hubris.network/application/o/device/",
+            "Scope": "openid profile email offline_access",
+            "UseIDToken": false
+        }
+    },
+    "PKCEAuthorizationFlow": {
+        "ProviderConfig": {
+            "ClientID": "netbird-dashboard",
+            "ClientSecret": "{{AUTHENTIK_CLIENT_SECRET}}",
+            "Audience": "netbird-dashboard",
+            "Domain": "auth.hubris.network",
+            "TokenEndpoint": "https://auth.hubris.network/application/o/token/",
+            "AuthorizationEndpoint": "https://auth.hubris.network/application/o/authorize/",
+            "Scope": "openid profile email offline_access",
+            "UseIDToken": false,
+            "RedirectURLs": ["http://localhost:53000/"]
+        }
+    }
+}