[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.in

@@ -52,6 +52,7 @@ rapidfuzz                               # Fuzzy string matching
 sentry-sdk                              # Error reporting (optional)
 setuptools                              # Standard dependency
 tablib[xls,xlsx,yaml]                   # Support for XLS and XLSX formats
+tqdm                                    # Progress bars for CLI
 weasyprint                              # PDF generation
 whitenoise                              # Enhanced static file serving