Move document type from document to document version

Follow the same pattern used for classification: document type now lives
exclusively on DocumentVersion. A migration copies existing values from
documents to their versions. The document filter uses a subquery on the
latest version. All three API surfaces (GraphQL, MCP, CLI), resolvers,
frontend, and e2e tests are updated accordingly.

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

pkg/coredata/migrations/20260331T120000Z.sql

@@ -0,0 +1,12 @@
+-- Add document_type column to document_versions, copying from parent document.
+ALTER TABLE document_versions ADD COLUMN document_type document_type NOT NULL DEFAULT 'OTHER';
+
+UPDATE document_versions dv
+SET document_type = d.document_type
+FROM documents d
+WHERE dv.document_id = d.id;
+
+ALTER TABLE document_versions ALTER COLUMN document_type DROP DEFAULT;
+
+-- TODO: drop the document_type column from documents.
+ALTER TABLE documents ALTER COLUMN document_type SET DEFAULT 'OTHER';