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.Scheduler.Storage.Mongo/Repositories/IRunRepository.cs

@@ -26,4 +26,10 @@ public interface IRunRepository
         RunQueryOptions? options = null,
         IClientSessionHandle? session = null,
         CancellationToken cancellationToken = default);
+
+    Task<IReadOnlyList<Run>> ListByStateAsync(
+        RunState state,
+        int limit = 50,
+        IClientSessionHandle? session = null,
+        CancellationToken cancellationToken = default);
 }

src/StellaOps.Scheduler.Storage.Mongo/Repositories/RunRepository.cs

@@ -150,4 +150,27 @@ internal sealed class RunRepository : IRunRepository
         var documents = await find.ToListAsync(cancellationToken).ConfigureAwait(false);
         return documents.Select(RunDocumentMapper.FromBsonDocument).ToArray();
     }
+
+    public async Task<IReadOnlyList<Run>> ListByStateAsync(
+        RunState state,
+        int limit = 50,
+        IClientSessionHandle? session = null,
+        CancellationToken cancellationToken = default)
+    {
+        if (limit <= 0)
+        {
+            throw new ArgumentOutOfRangeException(nameof(limit), limit, "Limit must be greater than zero.");
+        }
+
+        var filter = Filter.Eq("state", state.ToString().ToLowerInvariant());
+        var find = session is null
+            ? _collection.Find(filter)
+            : _collection.Find(session, filter);
+
+        find = find.Sort(Sort.Ascending("createdAt"));
+        find = find.Limit(limit);
+
+        var documents = await find.ToListAsync(cancellationToken).ConfigureAwait(false);
+        return documents.Select(RunDocumentMapper.FromBsonDocument).ToArray();
+    }
 }