feat(appgate): apps with their own login can skip the node login

Some apps carry a complete account system and are broken by an upstream
challenge: git clients speak basic-auth (not browser cookies), and a
BTCPay checkout link handed to a customer must open for that customer.
Both were behind the gate's login page — the "non-browser clients need an
access token" gap disclosed in five consecutive releases.

- New manifest port policy `auth: open`: the daemon still fronts the port
  exactly like `gated` (loopback pin, external binds, frame-header fixes,
  app-down retry page, Tor upstream) but serves it without the login
  challenge. Requires auth_rationale, same burden of proof as `none`.
  Gitea 3001 and BTCPay 23000 declare it.
- Runtime operator override per app (security.set-app-gate → app-configs/
  <id>.json "gateEnabled"), surfaced as Settings → app → Access control.
  Wins over the manifest in both directions and applies on the next
  request — no restart, and it works today on catalog-covered apps whose
  signed manifest still says `gated`.
- The gate resolves policy per-request from the live port map, so a
  toggle takes effect without waiting for the 60s rebind sweep. "Off"
  never releases the port: gated apps are loopback-pinned, so releasing
  would strand them, not open them.
- security.app-gate-status now reports gate_enabled + any override.
- New guard test pins the `auth: open` set (both entries reviewed); the
  `auth: none` count moves 25 → 26, absorbing pre-existing drift from the
  phoenixd onboarding (loopback JSON API with its own generated password).
- Docs: the manifest spec's ports row documented only host/container/
  protocol — bind, auth, auth_rationale and session_passthrough were
  undocumented. Added a full "Ports & the app gate" section plus a
  developer-guide entry telling app authors to enforce their own auth
  regardless, since the operator can flip the gate either way.

Verified live on archi-dev-box from an external address: gated → 401 gate
page; override off → Gitea 200 own page, BTCPay 302 to its own login,
git-over-HTTP info/refs 200; override on → 401 again; clear → default.

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

apps/btcpay-server/manifest.yml

@@ -46,7 +46,17 @@ app:
       container: 49392
       protocol: tcp
       bind: 127.0.0.1
-      auth: gated
+      # open, not gated: BTCPay has its own account system, and its public
+      # surfaces (checkout/invoice pages, payment buttons, webhooks) must be
+      # reachable by anonymous payers and machines — a dashboard login in
+      # front of a checkout link breaks the product. The gate still fronts
+      # the port; the operator can force the dashboard login back on from
+      # Settings → BTCPay Server → Access control.
+      auth: open
+      auth_rationale: >-
+        BTCPay enforces its own login for administration, and its checkout,
+        invoice and webhook endpoints are designed to be reached by
+        anonymous payers and payment processors.
 
   volumes:
     - type: bind