feat(security): DashboardTags on API-key constraints; sessions inherit owner tags (SEC-25 groundwork)

Adds a dashboard event-visibility tag to ApiKeyConstraints, riding in the existing
constraints JSON blob so no auth-store schema migration is needed (design
docs/plans/2026-07-10-dashboard-session-acl-tst15.md sections 3/3.1, open call
settled per its own recommendation). The tag is visibility-only: no read, write,
browse, or subscribe path consults it, and HasRead/HasWriteConstraints ignore it.

GatewaySession gains an immutable, ordinal-ignore-case Tags set stamped at
construction from the owning API key, forwarded by MxAccessGatewayService.OpenSession
from the resolved ApiKeyIdentity — never from the wire request, so a client cannot
label its own session with another tenant's tag. ISessionManager gains a tag-carrying
OpenSessionAsync overload whose default implementation forwards to the tagless one, so
an implementation that does not model tags opens an untagged (least visible) session.

apikey create-key gains --dashboard-tags team-a,team-b (repeatable, trimmed,
de-duplicated; an empty segment is rejected rather than dropped) and list-keys prints
the tags column. No enforcement yet — the EventsHub ACL that consumes the tag is a
later change.

src/ZB.MOM.WW.MxGateway.Server/Grpc/MxAccessGatewayService.cs

@@ -32,11 +32,17 @@ public sealed class MxAccessGatewayService(
         try
         {
             requestValidator.ValidateOpenSession(request);
+
+            // The session's owner id and its dashboard-visibility tags both come from the resolved
+            // API key identity, never from the request: the key is the tenant principal, so a
+            // client cannot label its own session with another tenant's tag (SEC-25).
+            ApiKeyIdentity? owner = identityAccessor.Current;
             GatewaySession session = await sessionManager
                 .OpenSessionAsync(
                     SessionOpenRequest.FromContract(request),
                     ResolveClientIdentity(),
-                    identityAccessor.Current?.KeyId,
+                    owner?.KeyId,
+                    owner?.EffectiveConstraints.DashboardTags,
                     context.CancellationToken)
                 .ConfigureAwait(false);