Performance: Stream JSON during import for memory improvements (#12276)

* Perf: stream manifest parsing with ijson in document_importer

Replace bulk json.load of the full manifest (which materializes the
entire JSON array into memory) with incremental ijson streaming.
Eliminates self.manifest entirely — records are never all in memory
at once.

- Add ijson>=3.2 dependency
- New module-level iter_manifest_records() generator
- load_manifest_files() collects paths only; no parsing at load time
- check_manifest_validity() streams without accumulating records
- decrypt_secret_fields() streams each manifest to a .decrypted.json
  temp file record-by-record; temp files cleaned up after file copy
- _import_files_from_manifest() collects only document records (small
  fraction of manifest) for the tqdm progress bar

Measured on 200 docs + 200 CustomFieldInstances:
- Streaming validation: peak memory 3081 KiB -> 333 KiB (89% reduction)
- Stream-decrypt to file: peak memory 3081 KiB -> 549 KiB (82% reduction)

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

* Perf: slim dict in _import_files_from_manifest, discard fields

When collecting document records for the file-copy step, extract only
the 4 keys the loop actually uses (pk + 3 exported filename keys) and
discard the full fields dict (content, checksum, tags, etc.).

Peak memory for the document-record list: 939 KiB -> 375 KiB (60% reduction).
Wall time unchanged.

src/documents/tests/test_management_importer.py

@@ -119,15 +119,22 @@ class TestCommandImport(
             # No read permissions
             original_path.chmod(0o222)
 
+            manifest_path = Path(temp_dir) / "manifest.json"
+            manifest_path.write_text(
+                json.dumps(
+                    [
+                        {
+                            "model": "documents.document",
+                            EXPORTER_FILE_NAME: "original.pdf",
+                            EXPORTER_ARCHIVE_NAME: "archive.pdf",
+                        },
+                    ],
+                ),
+            )
+
             cmd = Command()
             cmd.source = Path(temp_dir)
-            cmd.manifest = [
-                {
-                    "model": "documents.document",
-                    EXPORTER_FILE_NAME: "original.pdf",
-                    EXPORTER_ARCHIVE_NAME: "archive.pdf",
-                },
-            ]
+            cmd.manifest_paths = [manifest_path]
             cmd.data_only = False
             with self.assertRaises(CommandError) as cm:
                 cmd.check_manifest_validity()
@@ -296,7 +303,7 @@ class TestCommandImport(
         (self.dirs.scratch_dir / "manifest.json").touch()
 
         # We're not building a manifest, so it fails, but this test doesn't care
-        with self.assertRaises(json.decoder.JSONDecodeError):
+        with self.assertRaises(CommandError):
             call_command(
                 "document_importer",
                 "--no-progress-bar",
@@ -325,7 +332,7 @@ class TestCommandImport(
         )
 
         # We're not building a manifest, so it fails, but this test doesn't care
-        with self.assertRaises(json.decoder.JSONDecodeError):
+        with self.assertRaises(CommandError):
             call_command(
                 "document_importer",
                 "--no-progress-bar",