fix(backup): make .picpeak roundtrip work on Postgres

Two Postgres-only bugs found by a live docker-pg roundtrip (SQLite tests
passed because neither reproduces on SQLite):

- Export: knex `.stream()` pulls in the optional `pg-query-stream` module
  (not bundled) and throws on pg. Switched to a plain per-table `select`
  — works on both engines, no new dependency. Rows are DB metadata
  (blobs live under files/), so holding a table in memory is fine.
- Import: the pg driver returns json/jsonb columns as parsed JS values,
  so re-inserting a scalar like the string "PicPeak" sent it unquoted and
  pg rejected it ("invalid input syntax for type json"). Now introspects
  each table's json/jsonb columns and re-serialises those values before
  insert (pg only; SQLite stores json as TEXT and round-trips as-is).

Verified end-to-end on docker Postgres: export 85 tables, full-override
import, current account preserved, post-backup data removed.

backend/src/services/picpeakImportService.js

@@ -99,6 +99,30 @@ async function reinjectCurrentAdmin(trx, currentAdmin) {
   }
 }
 
+// The json/jsonb columns of a table (Postgres only). The pg driver returns
+// jsonb as parsed JS values, so on re-insert they must be serialised back to
+// valid JSON text — otherwise a scalar like the string "PicPeak" is sent
+// unquoted and pg rejects it ("invalid input syntax for type json").
+async function jsonColumnsFor(trx, table) {
+  if (!isPostgres()) return new Set();
+  const res = await trx.raw(
+    "SELECT column_name FROM information_schema.columns WHERE table_schema = 'public' AND table_name = ? AND data_type IN ('json', 'jsonb')",
+    [table]
+  );
+  return new Set(res.rows.map((r) => r.column_name));
+}
+
+function serialiseJsonColumns(rows, jsonCols) {
+  if (!jsonCols.size) return rows;
+  return rows.map((row) => {
+    const out = { ...row };
+    for (const col of jsonCols) {
+      if (out[col] !== undefined && out[col] !== null) out[col] = JSON.stringify(out[col]);
+    }
+    return out;
+  });
+}
+
 // Whole-DB replace in one transaction with FK enforcement suspended (pg:
 // session_replication_role=replica on the trx connection, reset before commit;
 // sqlite: defer_foreign_keys so checks run at commit). knex_migrations is never
@@ -113,7 +137,9 @@ async function replaceAllTables(tables, dataDir, currentAdmin) {
     }
     for (const table of tables) {
       const rows = parseNdjson(path.join(dataDir, `${table}.ndjson`));
-      if (rows.length) await trx.batchInsert(table, rows, 100);
+      if (!rows.length) continue;
+      const jsonCols = await jsonColumnsFor(trx, table);
+      await trx.batchInsert(table, serialiseJsonColumns(rows, jsonCols), 100);
     }
 
     await reinjectCurrentAdmin(trx, currentAdmin);