Add conditional-request support to PyPI metadata APIs

[dkliban]

Summary

Add Last-Modified / If-Modified-Since conditional request support to the Simple API and JSON Metadata API for improved cache efficiency. Also adds ETag, Cache-Control, and If-None-Match support to the JSON Metadata API which previously had none.

Changes:

• Extract _get_repo_version() helper to share repo version resolution between _etag_func and new _last_modified_func
• Add last_modified_func=_last_modified_func to @condition decorators on SimpleView.list and SimpleView.retrieve
• Add @condition(etag_func=_etag_func, last_modified_func=_last_modified_func) and @cache_control(max_age=900, public=True) to MetadataView.retrieve

Security: Authorization is checked before conditional request handling because DRF's initial() (permissions) runs before the view method where @condition operates.

Closes: #1338

Test plan

• test_simple_last_modified_header — Last-Modified present on Simple API responses
• test_simple_if_modified_since_304 — If-Modified-Since with matching timestamp returns 304
• test_simple_if_modified_since_old_timestamp_200 — old timestamp returns 200
• test_metadata_conditional_request_headers — ETag, Last-Modified, Cache-Control on metadata
• test_metadata_etag_conditional_request — If-None-Match 304/200
• test_metadata_if_modified_since_304 — If-Modified-Since 304
• test_metadata_if_modified_since_old_timestamp_200 — old timestamp returns 200
• test_unauthenticated_gets_401_not_304 — unauthenticated client gets 401, not 304, with conditional headers
• All 4 existing cache tests still pass

[jobselko]

@dkliban Please tag me as a reviewer once this is ready (maybe it already is?)