Performance: Further export memory improvements (#12273)

* Perf: streaming manifest writer for document exporter (Phase 3)

Replaces the in-memory manifest dict accumulation with a
StreamingManifestWriter that writes records to manifest.json
incrementally, keeping only one batch resident in memory at a time.

Key changes:
- Add StreamingManifestWriter: writes to .tmp atomically, BLAKE2b
  compare for --compare-json, discard() on exception
- Add _encrypt_record_inline(): per-record encryption replacing the
  bulk encrypt_secret_fields() call; crypto setup moved before streaming
- Add _write_split_manifest(): extracted per-document manifest writing
- Refactor dump(): non-doc records streamed during transaction, documents
  accumulated then written after filenames are assigned
- Upgrade check_and_write_json() from MD5 to BLAKE2b
- Remove encrypt_secret_fields() and unused itertools.chain import
- Add profiling marker to pyproject.toml

Measured improvement (200 docs + 200 CustomFieldInstances, same
dump() code path, only writer differs):
- Peak memory: ~50% reduction
- Memory delta: ~70% reduction
- Wall time and query count: unchanged

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* Refactor: O(1) lookup table for CRYPT_FIELDS in per-record encryption

Add CRYPT_FIELDS_BY_MODEL to CryptMixin, derived from CRYPT_FIELDS at
class definition time. _encrypt_record_inline() now does a single dict
lookup instead of a linear scan per record, eliminating the loop and
break pattern.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/documents/tests/test_management_exporter.py

@@ -753,6 +753,31 @@ class TestExportImport(
             call_command("document_importer", "--no-progress-bar", self.target)
             self.assertEqual(Document.objects.count(), 4)
 
+    def test_folder_prefix_with_split(self) -> None:
+        """
+        GIVEN:
+            - Request to export documents to directory
+        WHEN:
+            - Option use_folder_prefix is used
+            - Option split manifest is used
+        THEN:
+            - Documents can be imported again
+        """
+        shutil.rmtree(Path(self.dirs.media_dir) / "documents")
+        shutil.copytree(
+            Path(__file__).parent / "samples" / "documents",
+            Path(self.dirs.media_dir) / "documents",
+        )
+
+        self._do_export(use_folder_prefix=True, split_manifest=True)
+
+        with paperless_environment():
+            self.assertEqual(Document.objects.count(), 4)
+            Document.objects.all().delete()
+            self.assertEqual(Document.objects.count(), 0)
+            call_command("document_importer", "--no-progress-bar", self.target)
+            self.assertEqual(Document.objects.count(), 4)
+
     def test_import_db_transaction_failed(self) -> None:
         """
         GIVEN: