Add document classification filter

Add classification as a filter-only field on documents, resolved from
the latest document version. Expose in GraphQL, MCP, and document list
UI with a selector alongside the document type filter.

Signed-off-by: Sacha Al Himdani <sacha@getprobo.com>

pkg/server/api/console/v1/schema.graphql

@@ -1634,6 +1634,7 @@ input ControlFilter {
 input DocumentFilter {
     query: String
     documentTypes: [DocumentType!]
+    classifications: [DocumentClassification!]
     status: [DocumentStatus!]
 }