Bump pyrefly from 0.50.0 to 0.51.0 in /backend (#1549)

Bumps [pyrefly](https://github.com/facebook/pyrefly) from 0.50.0 to
0.51.0.
<details>
<summary>Commits</summary>
<ul>
<li><a
href="https://github.com/facebook/pyrefly/commit/1877edc64543bcc795584fa395d987eda5b7840b"><code>1877edc</code></a>
Update pyrefly version]</li>
<li><a
href="https://github.com/facebook/pyrefly/commit/cb5f5d7ba54727f39ec180b69201e013e680b72c"><code>cb5f5d7</code></a>
Make Unions take less space</li>
<li><a
href="https://github.com/facebook/pyrefly/commit/8b5434f1c5fc9dd1db9e7709525252016e98a7ac"><code>8b5434f</code></a>
Add <code>openlibrary</code> back to the blocklist</li>
<li><a
href="https://github.com/facebook/pyrefly/commit/87f0990a715d4f086e9332dae4a575d40896800e"><code>87f0990</code></a>
Fix pyrefly buck/cargo config</li>
<li><a
href="https://github.com/facebook/pyrefly/commit/ddd20cc2e9b3074231a7663e573d9486e79faf8c"><code>ddd20cc</code></a>
Remove old get() method from LookupExport trait</li>
<li><a
href="https://github.com/facebook/pyrefly/commit/e2aed196e7bdb91992c47fae1bdcd1be58af61d9"><code>e2aed19</code></a>
Convert alt/class/django.rs to use new LookupExport API</li>
<li><a
href="https://github.com/facebook/pyrefly/commit/0dcfcaef770aca34ac7b8972c94267059ba5771c"><code>0dcfcae</code></a>
Convert alt/attr.rs to use new LookupExport API</li>
<li><a
href="https://github.com/facebook/pyrefly/commit/bcf03acf006cb5d95e7dbb29801fe24a3e2fcaa8"><code>bcf03ac</code></a>
Convert binding/scope.rs to use new LookupExport API</li>
<li><a
href="https://github.com/facebook/pyrefly/commit/7092671918b2c8997447f54b5cbf91c2cc332c4f"><code>7092671</code></a>
Convert binding/bindings.rs to use new LookupExport API</li>
<li><a
href="https://github.com/facebook/pyrefly/commit/971a1154809f79860e2fa76861a7bd6d1834b520"><code>971a115</code></a>
Convert binding/stmt.rs to use new LookupExport API</li>
<li>Additional commits viewable in <a
href="https://github.com/facebook/pyrefly/compare/0.50.0...0.51.0">compare
view</a></li>
</ul>
</details>
<br />


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backend/pyproject.toml

@@ -55,7 +55,7 @@ dev = [
     "mypy==1.19.0",
     "pluggy==1.6.0",
     "pylint==4.0.2",
-    "pyrefly==0.50.0",
+    "pyrefly==0.51.0",
     "pytest-asyncio==1.3.0",
     "pytest-cov==7.0.0",
     "pytest-xdist==3.8.0",