feat: add pypi remote type with URL rewriting and basic auth

- Add 'pypi' package type to config.py; simple/ paths are mutable by default
- Refactor content-type detection into _get_content_type() helper; add .whl
- Add _resolve_content() which rewrites files host URLs in simple index HTML
  to go through the proxy (pypi_files_url / pypi_files_remote config keys),
  and returns text/html content-type for simple index responses
- Add basic auth support for non-Docker remotes (username + password/token
  in remote config); thread auth through _upstream_reachable and
  check_upstream_changed so mutable TTL checks also authenticate
- Add 'pypi' remote (pypi.org simple index) and 'pypi-files' remote
  (files.pythonhosted.org) to remotes.yaml; add 'pypi-gitea' example for
  Gitea package registries where index and files share the same base URL
- Add unit tests: simple index URL rewriting, HTML content-type, .whl/.tar.gz
  content-types, mutable index detection, and immutable pattern enforcement

tests/conftest.py

@@ -72,6 +72,25 @@ TEST_REMOTES = {
             "package": "generic",
             "cache": {"immutable_ttl": 0, "mutable_ttl": 0},
         },
+        "pypi-test": {
+            "base_url": "https://pypi.org",
+            "type": "remote",
+            "package": "pypi",
+            "pypi_files_url": "https://files.pythonhosted.org",
+            "pypi_files_remote": "pypi-files-test",
+            "cache": {"immutable_ttl": 0, "mutable_ttl": 600},
+        },
+        "pypi-files-test": {
+            "base_url": "https://files.pythonhosted.org",
+            "type": "remote",
+            "package": "generic",
+            "immutable_patterns": [
+                "packages/.*\\.whl$",
+                "packages/.*\\.whl\\.metadata$",
+                "packages/.*\\.tar\\.gz$",
+            ],
+            "cache": {"immutable_ttl": 0, "mutable_ttl": 0},
+        },
     }
 }