fix(migration): 7 critical fixes for Authority migration safety and data integrity

- Fix #2: _executeStatements fallback now batches transactions (150/batch)
  and reorders upsert-before-delete to prevent data loss on payload overflow
- Fix #3: Read/write migratedToAuthority marker in chat_metadata to prevent
  re-overwriting from legacy sources after Authority migration
- Fix #1: Add OPFS → Authority migration channel (exportOpfsSnapshotForChat,
  maybeImportLegacyOpfsSnapshotToLocalStore) inserted before IndexedDB in
  migration chain
- Fix #4: Mark runtimeVectorIndexState dirty with triviumRebuildRequired
  and trigger submitAuthorityVectorRebuildJob after all three migration paths
- Fix #5: Dual-save safety snapshots to Authority blob; rollback can now
  recover from blob when local IndexedDB snapshot is unavailable
- Fix #6: Add isEmptyCheck detail and console.warn for near-empty stores
  to help diagnose residual vs real data in store-not-empty skips
- Fix #7: Add overflow warning logs and sessionStorage persistence for
  Authority offline queue max-items/max-bytes exceeded events

sync/bme-opfs-store.js

@@ -1320,6 +1320,12 @@ class LegacyOpfsGraphStore {
           edges: emptyStatus.edges,
           tombstones: emptyStatus.tombstones,
         },
+        isEmptyCheck: {
+          empty: false,
+          nodes: emptyStatus.nodes,
+          edges: emptyStatus.edges,
+          tombstones: emptyStatus.tombstones,
+        },
         migrationCompletedAt: 0,
         migrationSource,
         legacyRetentionUntil,
@@ -2707,6 +2713,12 @@ export class OpfsGraphStore {
           edges: emptyStatus.edges,
           tombstones: emptyStatus.tombstones,
         },
+        isEmptyCheck: {
+          empty: false,
+          nodes: emptyStatus.nodes,
+          edges: emptyStatus.edges,
+          tombstones: emptyStatus.tombstones,
+        },
         migrationCompletedAt: 0,
         migrationSource,
         legacyRetentionUntil,