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/mcp/v1/specification.yaml

@@ -5374,6 +5374,11 @@ components:
               items:
                 $ref: "#/components/schemas/DocumentType"
               description: Document types
+            classifications:
+              type: array
+              items:
+                $ref: "#/components/schemas/DocumentClassification"
+              description: Document classifications
 
     ListDocumentsOutput:
       type: object