feat(pkgsite): adapt v1 API and redis rate limiting

[garrettladley]

• switches upstream requests to pkg.go.dev/v1 with filter, pagination, ambiguity, and structured error handling
• exposes typed v1 package, module, list, vulnerability, and imported-by fields through primitive JSON_DATA and pkgsite_explain
• coordinates the upstream 45 qps courtesy limit through shared kv.Store windows with context-aware waiting, retry accounting, and no 429 caching
• documents the v1 filter syntax and redis-backed caching and rate-limiting behavior

[greptile-apps]

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