Resolve Server-002, -004, -005, -006 code-review findings

Server-002: the gateway never terminated leftover MxGateway.Worker.exe
processes at startup, contradicting gateway.md and CLAUDE.md. Added
IRunningProcessInspector/SystemRunningProcessInspector, OrphanWorkerTerminator,
and OrphanWorkerCleanupHostedService (best-effort, runs before sessions are
accepted); updated gateway.md to describe the implemented behavior.

Server-004: API-key scopes were persisted verbatim with no validation. Added
GatewayScopes.All/IsKnown; the CLI parser and dashboard create path now
reject unknown scope strings.

Server-005: a non-SqlException/InvalidOperationException fault on the initial
Galaxy hierarchy load faulted the BackgroundService. ExecuteAsync now catches
all non-cancellation exceptions on first load and RefreshCoreAsync broadens
its catch so the cache records Stale/Unavailable instead.

Server-006: OpenSessionAsync incremented the open-sessions gauge before
alarm auto-subscribe; an auto-subscribe failure leaked the gauge. The catch
path now calls SessionRemoved() when the gauge was incremented.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/MxGateway.Server/Workers/OrphanWorkerCleanupHostedService.cs

@@ -0,0 +1,30 @@
+namespace MxGateway.Server.Workers;
+
+/// <summary>
+///     Hosted service that terminates leftover MXAccess worker processes once on
+///     gateway startup, before the server begins accepting sessions.
+/// </summary>
+public sealed class OrphanWorkerCleanupHostedService(
+    OrphanWorkerTerminator terminator,
+    ILogger<OrphanWorkerCleanupHostedService> logger) : IHostedService
+{
+    /// <inheritdoc />
+    public Task StartAsync(CancellationToken cancellationToken)
+    {
+        try
+        {
+            terminator.TerminateOrphans();
+        }
+        catch (Exception exception)
+        {
+            // Orphan cleanup is best-effort; a failure here must not prevent
+            // the gateway from starting.
+            logger.LogWarning(exception, "Orphan worker cleanup failed on startup.");
+        }
+
+        return Task.CompletedTask;
+    }
+
+    /// <inheritdoc />
+    public Task StopAsync(CancellationToken cancellationToken) => Task.CompletedTask;
+}