feat: Implement Scheduler Worker Options and Planner Loop

- Added `SchedulerWorkerOptions` class to encapsulate configuration for the scheduler worker.
- Introduced `PlannerBackgroundService` to manage the planner loop, fetching and processing planning runs.
- Created `PlannerExecutionService` to handle the execution logic for planning runs, including impact targeting and run persistence.
- Developed `PlannerExecutionResult` and `PlannerExecutionStatus` to standardize execution outcomes.
- Implemented validation logic within `SchedulerWorkerOptions` to ensure proper configuration.
- Added documentation for the planner loop and impact targeting features.
- Established health check endpoints and authentication mechanisms for the Signals service.
- Created unit tests for the Signals API to ensure proper functionality and response handling.
- Configured options for authority integration and fallback authentication methods.

src/StellaOps.Authority/StellaOps.Auth.Abstractions/StellaOpsServiceIdentities.cs

@@ -14,4 +14,9 @@ public static class StellaOpsServiceIdentities
     /// Service identity used by Cartographer when constructing and maintaining graph projections.
     /// </summary>
     public const string Cartographer = "cartographer";
+
+    /// <summary>
+    /// Service identity used by Vuln Explorer when issuing scoped permalink requests.
+    /// </summary>
+    public const string VulnExplorer = "vuln-explorer";
 }