✨(backend) add file upload feature & tests

For the coming features we will need to store files on the meet side.
(for instance user backgrounds).

This commits adds a new Model to manage files, and the associated
serializers & viewsets. All are tested.

This work was heavily inspired by the work done by our friends at
https://github.com/suitenumerique/drive
It build on the same architecture design (upload directly to S3 but
download goes through our proxy), but model is much much simplier
(no folders, no file sharing, etc.).

src/backend/core/tasks/_task.py

@@ -0,0 +1,49 @@
+# ruff: noqa: PLC0415
+
+from django.conf import settings
+
+
+def task(*d_args, **d_kwargs):
+    """
+    Decorator compatible with Celery's @app.task, but works without Celery.
+
+    If Celery is available, returns a real Celery task.
+    If not, returns the original function and provides `.delay()`/`.apply_async()`
+    as synchronous fallbacks (so existing call sites don't break).
+
+    Notes:
+        Mostly LLM-generated.
+    """
+
+    def _fallback_wrap(func):
+        def delay(*args, **kwargs):
+            return func(*args, **kwargs)
+
+        def apply_async(args=None, kwargs=None, **_options):
+            return func(*(args or ()), **(kwargs or {}))
+
+        func.delay = delay
+        func.apply_async = apply_async
+        return func
+
+    # Handle bare decorator usage: @task
+    if len(d_args) == 1 and callable(d_args[0]) and not d_kwargs:
+        func = d_args[0]
+        if settings.CELERY_ENABLED:
+            from meet.celery_app import app as _celery_app
+
+            return _celery_app.task(func)
+        return _fallback_wrap(func)
+
+    # Handle parameterized usage: @task(...), e.g. @task(bind=True)
+    def _decorate(func):
+        if settings.CELERY_ENABLED:
+            from meet.celery_app import app as _celery_app
+
+            return _celery_app.task(*d_args, **d_kwargs)(func)
+        return _fallback_wrap(func)
+
+    return _decorate
+
+
+__all__ = ("task",)

src/backend/core/tasks/file.py

@@ -0,0 +1,36 @@
+"""
+Tasks related to files.
+"""
+
+import logging
+
+from django.core.files.storage import default_storage
+
+from core.models import File
+from core.tasks._task import task
+
+logger = logging.getLogger(__name__)
+
+
+@task
+def process_file_deletion(file_id):
+    """
+    Process the deletion of a file.
+    Definitely delete it in the database.
+    Delete the files from the storage.
+    """
+    logger.info("Processing item deletion for %s", file_id)
+    try:
+        file = File.objects.get(id=file_id)
+    except File.DoesNotExist:
+        logger.error("Item %s does not exist", file_id)
+        return
+
+    if file.hard_deleted_at is None:
+        logger.error("To process an item deletion, it must be hard deleted first.")
+        return
+
+    logger.info("Deleting file %s", file.file_key)
+    default_storage.delete(file.file_key)
+
+    file.delete()