Aspire hosting resource and a standalone dashboard host

The Aspire dashboard is the DX every .NET developer now expects: run one container, point the apps at it with an environment variable, open a browser. Quartz's dashboard is a package you host in your own ASP.NET Core application, which is right for a product and wrong for an operator who has six worker services and no web app.

Shape

• Quartz.Aspire.Hosting (this one does take Aspire.Hosting): builder.AddQuartzDashboard() adds a dashboard resource that fronts every project referencing the same database connection (store-attached) and every project given a WithReference(dashboard) (agent), wiring the endpoint and token through environment variables.
• A standalone dashboard host (container image and dotnet tool) configured by Quartz__Dashboard__Targets__…: HTTP targets, attached stores, an agent listener; authentication by API key or OIDC, never anonymous by default.

Done means

• The Aspire example's AppHost shows the dashboard beside the worker; the standalone host runs from configuration alone; both documented.

Depends on the fleet-pane issue.

Part of #3387 — see the design record there for how this fits the whole.