Fix proactive dependency caching HTTPS redirect issue

When background threads fetch from our own proxy using the request's
base_url, it returns http:// but ingress requires https://. The 308
redirect was dropping trailing slashes, causing requests to hit the
frontend catch-all route instead of /pypi/simple/.

Force HTTPS explicitly in the background caching function to avoid
the redirect entirely.

backend/app/pypi_proxy.py

@@ -525,6 +525,9 @@ def _cache_dependency_background(
         normalized_name = re.sub(r'[-_.]+', '-', dep_name).lower()
 
         # First, get the simple index page to find available versions
+        # Use HTTPS explicitly to avoid redirect issues that can drop trailing slashes
+        if base_url.startswith('http://'):
+            base_url = 'https://' + base_url[7:]
         simple_url = f"{base_url}/pypi/simple/{normalized_name}/"
         logger.info(f"PyPI proxy: proactively caching {dep_name} (depth={depth})")