[refactor] Attachment images (#11961)

* Add new Attachment model fields:

- is_image
- thumbnail

* Cache if the attachment is an image

* Add new setting for controlling max upload size

* Validate uploaded attachment file

* Add tqdm for progress bars

* Refactor migrations

- Don't need is_image field
- Can introspect from the thumbnail

* Data migration for existing attachments

* Bump API version

* Update tests and validators

* Add "is_image" field to the Attachment model

* Offload to background task

* Implement unit tests

* Docs

* Add unit test for data migration

* Additional unit test

* Omit migration tests from code coverage

* Additional unit tests

src/backend/requirements-3.14.txt

@@ -2152,6 +2152,12 @@ tinyhtml5==2.1.0 \
     # via
     #   -c src/backend/requirements.txt
     #   weasyprint
+tqdm==4.67.3 \
+    --hash=sha256:7d825f03f89244ef73f1d4ce193cb1774a8179fd96f31d7e1dcde62092b960bb \
+    --hash=sha256:ee1e4c0e59148062281c49d80b25b67771a127c85fc9676d3be5f243206826bf
+    # via
+    #   -c src/backend/requirements.txt
+    #   -r src/backend/requirements.in
 typing-extensions==4.15.0 \
     --hash=sha256:0cea48d173cc12fa28ecabc3b837ea3cf6f38c6d1136f85cbaaf598984861466 \
     --hash=sha256:f0fa19c6845758ab08074a0cfa8b7aecb71c999ca73d62883bc25cc018c4e548