fix(security): harden external input handling

Summary

• Harden external payload normalization for domain webhooks and drops by replacing arbitrary string-to-atom conversion with explicit key whitelists.
• Normalize malformed OAuth registration metadata safely and prune stale OAuth registration rate-limit buckets.
• Share XML escaping/rendering helpers across Atom and RSS feeds, and centralize comma-separated query parsing.
• Codify the review learnings in the Blick Hive Elixir review skill.

Testing

• mix compile --warnings-as-errors
• mix test
• mix credo
• mix hex.audit
• git diff --check

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