fix(nat): strip only dashboard-issued Authorization credentials at NAT ingress

Since 2640b86 (v2.2.11, 2026-07-20) the NAT ingress deleted the entire
Authorization header, breaking self-hosted apps behind the tunnel that
authenticate with their own Bearer tokens (e.g. the Qinglong panel login
loop).

Strip only credentials issued by this dashboard and forward foreign ones:

- controller.IsDashboardCredential classifies an Authorization value as
  dashboard-issued via a signature-only JWT parser derived from the auth
  middleware (same key, HS256 pinned, claims validation deliberately
  skipped so an expired panel token is still recognized and kept out of
  the tunnel) or an api_tokens lookup for PAT-shaped values. Unknown PATs,
  foreign signatures, other schemes and missing values pass through. When
  classification cannot complete (parser not ready, DB nil or error) the
  verdict fails closed (strip) and the log carries a fixed reason only,
  never credential content.
- Both NAT capability variants (default, and agent-compat legacy /
  invalid-capability / post-consume paths) replace the unconditional
  Del("Authorization") with stripDashboardCredential. The rpc package
  receives the classifier through SetNATDashboardCredentialGate, the same
  injection pattern as SetMCPKillSwitchObserver; an unwired gate keeps the
  historical strip-everything behavior.

Tests: controller classification semantics (panel JWT, expired panel JWT
and panel PAT stripped; foreign-signed JWT, unknown nzp_ token, random
strings, non-Bearer schemes and missing header pass through; parser
unavailable, DB nil and DB error fail closed with credential-free logs)
plus rpc unit and flow coverage for both variants (dashboard credentials
never reach the forwarded bytes; foreign and missing Authorization are
forwarded untouched).

Compatibility: no API, database or frontend changes; NAT backends that do
not use Authorization behave exactly as before.

cmd/dashboard/rpc/nat.go

@@ -80,7 +80,14 @@ func ServeNAT(w http.ResponseWriter, r *http.Request, natConfig *model.NAT) {
 		return
 	}
 
-	// Authorization authenticates Dashboard access before NAT ingress; it must not become an origin credential for the configured NAT backend.
+	// Credential invariant at NAT ingress: credentials issued by this
+	// dashboard — the panel-signed JWT (verified by signature only, expiry
+	// deliberately ignored) and the panel API token — must never become origin
+	// credentials for the configured NAT backend. prepareNATCapability has
+	// already stripped every Authorization value IsDashboardCredential
+	// classified as dashboard-issued; whatever remains is either absent or a
+	// foreign credential, which is forwarded untouched because the NAT backend
+	// may authenticate with credentials of its own.
 	wWrapped, err := utils.NewRequestWrapper(r, w)
 	if err != nil {
 		return