⬆️(backend) upgrade docspec to v3.0.0 and adapt converter API

Summary

- Bump docspec Docker image from `2.6.3` to `3.0.0` and adapt
`DocSpecConverter` to the new API (raw body upload with explicit
`Content-Type`/`Accept` headers instead of multipart form)

Important

**The Docker image (`ghcr.io/docspecio/api:3.0.0`) must be updated
alongside the code changes.** The new request format is incompatible
with v2.x — deploying only the code without updating the image (or vice
versa) will break document conversion.

src/backend/core/tests/test_services_docspec_converter.py

@@ -110,8 +110,11 @@ def test_docspec_convert_success(mock_post, settings):
     # Verify the request was made correctly
     mock_post.assert_called_once_with(
         "http://docspec.test/convert",
-        headers={"Accept": mime_types.BLOCKNOTE},
-        files={"file": ("document.docx", docx_data, mime_types.DOCX)},
+        headers={
+            "Content-Type": mime_types.DOCX,
+            "Accept": mime_types.BLOCKNOTE,
+        },
+        data=docx_data,
         timeout=5,
         verify=False,
     )