feat(plantare): schema v12 — bilateral signed columns + repo

Adds the deferred bilateral columns to Plantares (plantare-bilateral.md
§"Schema delta"), all nullable/defaulted so v1 local rows coexist
untouched:

- pledgeId (the shared id both parties key their row by), debtorKey /
  creditorKey, debtorSignature / creditorSignature, movementId (the
  shared hand-over Movement for the provenance DAG), remoteState
  (proposed/accepted/declined — the handshake, distinct from status),
  returnKind (similar/workHours/other, default similar) + workHours.

Repo: createPlantare grows optional bilateral params; plantareByPledgeId
looks a row up by the shared id; applyPlantareSignatures counter-stamps
an incoming accept; setPlantareRemoteState records a decline. The
JSON codec carries every new field so they ride backups.

Versioned migration is guarded/idempotent like the others; schema v12
dumped, test helper regenerated, migration test covers v1..v11 → v12.
db + data + services 295/295 green.

apps/app_seeds/test/db/schema/schema.dart

@@ -15,6 +15,7 @@ import 'schema_v8.dart' as v8;
 import 'schema_v9.dart' as v9;
 import 'schema_v10.dart' as v10;
 import 'schema_v11.dart' as v11;
+import 'schema_v12.dart' as v12;
 
 class GeneratedHelper implements SchemaInstantiationHelper {
   @override
@@ -42,10 +43,12 @@ class GeneratedHelper implements SchemaInstantiationHelper {
         return v10.DatabaseAtV10(db);
       case 11:
         return v11.DatabaseAtV11(db);
+      case 12:
+        return v12.DatabaseAtV12(db);
       default:
         throw MissingSchemaException(version, versions);
     }
   }
 
-  static const versions = const [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11];
+  static const versions = const [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12];
 }