Implement Advisory Canonicalization and Backfill Migration

- Added AdvisoryCanonicalizer for canonicalizing advisory identifiers.
- Created EnsureAdvisoryCanonicalKeyBackfillMigration to populate advisory_key and links in advisory_raw documents.
- Introduced FileSurfaceManifestStore for managing surface manifests with file system backing.
- Developed ISurfaceManifestReader and ISurfaceManifestWriter interfaces for reading and writing manifests.
- Implemented SurfaceManifestPathBuilder for constructing paths and URIs for surface manifests.
- Added tests for FileSurfaceManifestStore to ensure correct functionality and deterministic behavior.
- Updated documentation for new features and migration steps.

src/Concelier/StellaOps.Concelier.WebService/Extensions/AdvisoryRawRequestMapper.cs

@@ -61,24 +61,26 @@ internal static class AdvisoryRawRequestMapper
             identifiersRequest.Primary);
 
         var linksetRequest = request.Linkset;
-        var linkset = new RawLinkset
-        {
-            Aliases = NormalizeStrings(linksetRequest?.Aliases),
-            PackageUrls = NormalizeStrings(linksetRequest?.PackageUrls),
-            Cpes = NormalizeStrings(linksetRequest?.Cpes),
-            References = NormalizeReferences(linksetRequest?.References),
-            ReconciledFrom = NormalizeStrings(linksetRequest?.ReconciledFrom),
-            Notes = NormalizeDictionary(linksetRequest?.Notes)
-        };
-
-        return new AdvisoryRawDocument(
-            tenant.Trim().ToLowerInvariant(),
-            source,
-            upstream,
-            content,
-            identifiers,
-            linkset);
-    }
+        var linkset = new RawLinkset
+        {
+            Aliases = NormalizeStrings(linksetRequest?.Aliases),
+            PackageUrls = NormalizeStrings(linksetRequest?.PackageUrls),
+            Cpes = NormalizeStrings(linksetRequest?.Cpes),
+            References = NormalizeReferences(linksetRequest?.References),
+            ReconciledFrom = NormalizeStrings(linksetRequest?.ReconciledFrom),
+            Notes = NormalizeDictionary(linksetRequest?.Notes)
+        };
+
+        return new AdvisoryRawDocument(
+            tenant.Trim().ToLowerInvariant(),
+            source,
+            upstream,
+            content,
+            identifiers,
+            linkset,
+            AdvisoryKey: string.Empty,
+            Links: ImmutableArray<RawLink>.Empty);
+    }
 
     internal static ImmutableArray<string> NormalizeStrings(IEnumerable<string>? values)
     {