✨(migration) add export_migration management command and export users

This is the first model to be exported. Dynamically export any existing
field of the model, this way this management command should be able to
run on any version.

Signed-off-by: Mathieu Agopian <mathieu@agopian.info>

CHANGELOG.md

@@ -72,6 +72,8 @@ and this project adheres to
   offline
 - 🐛(frontend) stop the service worker from caching the collaboration server's
   rest api
+- ✨(backend) export_migration management command to export data from an
+  instance to a JSONL file
 
 ### Changed
 

src/backend/core/management/commands/export_migration.py

@@ -0,0 +1,96 @@
+"""export_migration — export data as JSONL for migration between instances.
+
+Each output line is a self-contained JSON object:
+
+    {"model": "<app_label>.<model>", "pk": "<uuid>", "fields": { ... }}
+
+Models are written in dependency order. Fields and models that do not exist
+in the running version are skipped automatically, so the same script runs
+across multiple Docs versions.
+
+JSONL goes to stdout; progress and diagnostics go to stderr.
+
+Note: this script was written to be run from a version 3.4.2 or above. All
+models that are exported unconditionally are present in v3.4.2. Any model above
+v3.4.2 is exported conditionally (only if they exist).
+
+Usage:
+    python manage.py export_migration > migration.jsonl
+"""
+
+import json
+import sys
+import uuid
+from datetime import datetime
+
+from django.core.management.base import BaseCommand
+
+from core.models import User
+
+try:
+    from zoneinfo import ZoneInfo as _ZoneInfo
+except ImportError:
+    _ZoneInfo = None
+
+
+class _Encoder(json.JSONEncoder):
+    """Serialize types that the standard encoder cannot handle."""
+
+    def default(self, o):
+        if isinstance(o, uuid.UUID):
+            return str(o)
+        if isinstance(o, datetime):
+            return o.isoformat()
+        # timezone_field with use_pytz=False returns ZoneInfo objects;
+        # str() gives the IANA name (e.g. "Europe/Paris")
+        if _ZoneInfo and isinstance(o, _ZoneInfo):
+            return str(o)
+        return super().default(o)  # raises TypeError for anything unexpected
+
+
+def _write(stream, model_label, pk, fields):
+    """Emit one JSONL record."""
+    stream.write(
+        json.dumps(
+            {"model": model_label, "pk": str(pk), "fields": fields},
+            cls=_Encoder,
+        )
+    )
+    stream.write("\n")
+
+
+class Command(BaseCommand):
+    """Export data as JSONL for migration between instances."""
+
+    help = __doc__
+
+    def handle(self, *args, **options):
+        """Run the export, writing JSONL to stdout."""
+        self._export(sys.stdout)
+
+    def _export(self, stream):
+        self._export_users(stream)
+
+    def _progress(self, label, count, done=False):
+        """Overwrite the current stderr line with an incrementing count."""
+        ending = "\n" if done else ""
+        self.stderr.write(f"\r  {label}: {count}", ending=ending)
+        self.stderr.flush()
+
+    def _export_users(self, stream):
+        """
+        Export all concrete User fields.
+
+        values() returns exactly the fields that exist in the running schema,
+        so version-specific fields (e.g. is_first_connection, added in
+        migration 0030) are included or omitted automatically.
+        M2M fields (groups, user_permissions) are not returned by values().
+        """
+        self.stderr.write("Exporting users...")
+        count = 0
+        for user in User.objects.order_by("created_at").values().iterator():
+            pk = user.pop("id")
+            _write(stream, "core.user", pk, user)
+            count += 1
+            self._progress("users", count)
+        self._progress("users", count, done=True)