SOA as document: replace export with publish workflow

Statements of Applicability are no longer exported as one-off PDFs.
Instead, each SOA owns a persistent document that accumulates versions
over time, following the same publish/approve lifecycle as authored
documents.

Publishing without approvers publishes immediately; publishing with
approvers creates a draft pending approval via the existing quorum
system. SOAs can also store default approvers that are pre-populated in
the publish dialog.

The SOA is removed from the snapshot system — applicability statements
are now queried directly (snapshot_id IS NULL) rather than through
snapshot copies.

A standalone migration script (cmd/migrate-soa-snapshots-to-documents)
converts existing SOA snapshots into documents with proper ProseMirror
content, preserving version history and approval decisions.

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

pkg/server/api/trust/v1/graphql/trust_center.graphql

@@ -83,6 +83,10 @@ enum DocumentType
   REPORT @goEnum(value: "go.probo.inc/probo/pkg/coredata.DocumentTypeReport")
   TEMPLATE
     @goEnum(value: "go.probo.inc/probo/pkg/coredata.DocumentTypeTemplate")
+  STATEMENT_OF_APPLICABILITY
+    @goEnum(
+      value: "go.probo.inc/probo/pkg/coredata.DocumentTypeStatementOfApplicability"
+    )
 }
 
 type Document implements Node @nda {