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/coredata/document_write_mode.go

@@ -0,0 +1,48 @@
+// Copyright (c) 2026 Probo Inc <hello@getprobo.com>.
+//
+// Permission to use, copy, modify, and/or distribute this software for any
+// purpose with or without fee is hereby granted, provided that the above
+// copyright notice and this permission notice appear in all copies.
+//
+// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+// REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+// AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+// INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+// LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+// OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+// PERFORMANCE OF THIS SOFTWARE.
+
+package coredata
+
+import "fmt"
+
+type (
+	DocumentWriteMode string
+)
+
+const (
+	DocumentWriteModeAuthored  DocumentWriteMode = "AUTHORED"
+	DocumentWriteModeGenerated DocumentWriteMode = "GENERATED"
+)
+
+func (e DocumentWriteMode) IsValid() bool {
+	switch e {
+	case DocumentWriteModeAuthored, DocumentWriteModeGenerated:
+		return true
+	}
+	return false
+}
+
+func (e DocumentWriteMode) String() string { return string(e) }
+
+func (e *DocumentWriteMode) UnmarshalText(text []byte) error {
+	*e = DocumentWriteMode(text)
+	if !e.IsValid() {
+		return fmt.Errorf("%s is not a valid DocumentWriteMode", string(text))
+	}
+	return nil
+}
+
+func (e DocumentWriteMode) MarshalText() ([]byte, error) {
+	return []byte(e.String()), nil
+}