feat(security): opt-in authentication for /metrics (SEC-14)

New MxGateway:Metrics:RequireAuthentication (default false). When true,
/metrics requires an API key with the admin scope (verified through the
cached verifier and the shared API-key failure limiter) or a dashboard
login with a real dashboard role; anonymous localhost and
Authentication:Mode=Disabled do not qualify. An anonymous scrape gets a
plain 401 Bearer challenge, a throttled peer 429. /health stays anonymous
because the off-repo Overview dashboard polls /health/active.

MapZbMetrics returns IEndpointConventionBuilder, so the policy is attached
with RequireAuthorization.

docs/DesignDecisions.md

@@ -430,6 +430,41 @@ Suggested mapping:
 - Galaxy SQL metadata endpoints if added: `metadata:read`
 - worker shutdown diagnostics and key management: `admin`
 
+## Metrics Endpoint Authentication (SEC-14)
+
+Decision: `/metrics` stays anonymous by default and can be put behind
+authentication with `MxGateway:Metrics:RequireAuthentication=true`; `/health`
+stays anonymous unconditionally.
+
+- **Opt-in rather than on by default.** Existing Prometheus scrapers on the
+  deployed hosts scrape anonymously; flipping the default would silently break
+  them on upgrade. Hosts whose network exposure makes the counters sensitive
+  enable it explicitly.
+- **Health is never gated.** The family Overview dashboard (another repo) polls
+  `/health/active` without credentials, and liveness/readiness probes are
+  conventionally anonymous. Health responses carry status only.
+- **Credentials reuse existing plumbing.** One `MxGateway.Metrics` scheme
+  accepts an API key with the `admin` scope (verified through the same cached
+  `IApiKeyVerifier` and the same per-peer failure limiter as gRPC, so `/metrics`
+  is not a way around the gRPC brute-force throttle) or, when no authorization
+  header is sent, the dashboard login cookie with a real dashboard role. No new
+  credential type or store is introduced.
+- **`admin` scope, any dashboard role.** Metrics are an operator surface, so a
+  data-plane client key (`invoke:*`, `events:read`) is not enough. A dashboard
+  `Viewer` qualifies because every dashboard role already sees the same aggregate
+  counters on the dashboard; the exported metrics carry no session ids or tag
+  values.
+- **No environment bypass.** The requirement reads no options, so anonymous
+  localhost and `Authentication:Mode=Disabled` — both of which satisfy the
+  dashboard Viewer policy — do not satisfy it. Only dev-only
+  `Dashboard:DisableLogin` (which authenticates every request as an
+  Administrator, and is rejected in Production) lets an unauthenticated scrape
+  through.
+- **One scheme, plain 401.** Listing the cookie scheme on the policy directly
+  would challenge an anonymous scrape with a 302 redirect to `/login`; a single
+  scheme that falls back to the cookie internally keeps the challenge a `401`
+  with `WWW-Authenticate: Bearer`, which scrapers understand.
+
 ## Worker Process Identity
 
 Decision: run workers as the gateway service identity for v1.