🔒️(backend) prevnt file change post checks

Before this commit, post a file check, the policy could be
reused to change the verified file.
Now, files are uplaoded to a temporary location, then inside
a transaction that prevents concurrent calls, the file is
copied to its final destination and the checks are run on that one.
A new file can still be updated with the policy but it will never be read, etc.

As part of this change, all files in the new tmp directory
on s3 should have an expiration policy.

src/backend/core/models.py

@@ -954,6 +954,16 @@ class File(BaseModel):
 
         return f"{settings.FILE_UPLOAD_PATH}/{self.pk!s}"
 
+    @property
+    def temporary_key_base(self):
+        """Temporary key base used while upload is still pending."""
+        if not self.pk:
+            raise RuntimeError(
+                "The file instance must be saved before requesting a storage key."
+            )
+
+        return f"{settings.FILE_UPLOAD_TMP_PATH}/{self.pk!s}"
+
     @property
     def file_key(self):
         """Key used to store the file in object storage."""
@@ -962,6 +972,12 @@ class File(BaseModel):
         # leaking Personal Information in logs, etc.
         return f"{self.key_base}{extension!s}"
 
+    @property
+    def temporary_file_key(self):
+        """Temporary key used to upload the file before it is finalized."""
+        _, extension = splitext(self.filename)
+        return f"{self.temporary_key_base}{extension!s}"
+
     def get_abilities(self, user):
         """
         Compute and return abilities for a given user on the file.