partly or unimplemented features - now implemented

docs/modules/concelier/federation-operations.md

@@ -496,3 +496,89 @@ DSSE envelope contains:
 4. **Rotate signing keys periodically**
 5. **Audit import events**
 6. **Monitor for duplicate bundle imports**
+## Snapshot Pinning and Rollback
+
+> **Sprint:** SPRINT_20260208_035_Concelier_feed_snapshot_coordinator
+
+### Overview
+
+Snapshot pinning provides cross-instance coordination for federated deployments. It ensures that:
+
+- All federated sites can synchronize to a common snapshot version
+- Failed imports are automatically rolled back to the previous stable state
+- Concurrent snapshot operations are detected and prevented
+
+### Services
+
+The following services are registered by `AddConcelierFederationServices()`:
+
+| Service | Description |
+|---------|-------------|
+| `IFeedSnapshotPinningService` | Low-level snapshot pinning using SyncLedgerRepository |
+| `ISnapshotIngestionOrchestrator` | High-level orchestration with automatic rollback |
+
+### Automatic Rollback on Import Failure
+
+When importing a snapshot bundle, the `ISnapshotIngestionOrchestrator` provides:
+
+1. **Lock acquisition** - Prevents concurrent operations on the same source
+2. **Conflict detection** - Checks for cursor conflicts before proceeding
+3. **Pin-before-import** - Pins the snapshot ID before import begins
+4. **Automatic rollback** - On import failure, automatically reverts to previous state
+
+```csharp
+// Example usage in application code
+var result = await orchestrator.ImportWithRollbackAsync(
+    inputStream,
+    importOptions,
+    sourceId,
+    cancellationToken);
+
+if (!result.Success)
+{
+    if (result.WasRolledBack)
+    {
+        _logger.LogWarning(
+            "Import failed but rolled back to {SnapshotId}",
+            result.RolledBackToSnapshotId);
+    }
+}
+```
+
+### API Endpoints
+
+The snapshot pinning service is available through the existing feed snapshot endpoints:
+
+```
+POST /api/v1/feeds/snapshot/import
+```
+
+When the orchestrator is used, the response includes rollback information:
+
+```json
+{
+  "success": false,
+  "error": "Import failed: invalid bundle format",
+  "was_rolled_back": true,
+  "rolled_back_to_snapshot_id": "snapshot-2024-001"
+}
+```
+
+### Configuration
+
+Snapshot pinning uses the same `FederationOptions` as other federation features:
+
+```yaml
+Federation:
+  Enabled: true
+  SiteId: "site-us-west-1"  # Required for pinning coordination
+```
+
+### Monitoring
+
+Key metrics for snapshot pinning:
+
+- `snapshot_pin_success_total` - Successful pin operations
+- `snapshot_pin_failure_total` - Failed pin operations
+- `snapshot_rollback_total` - Rollback operations triggered
+- `snapshot_conflict_total` - Conflict detections