feat(search): allow collaborators to find posts by response text

[lol2x]

Summary

Extend the post search so collaborators and administrators can match against
posts.response text (the staff note set when changing a post's status).

This makes it possible to find posts by what was written when closing them —
e.g. retrieving every post marked "Shipped in v2.3.1". For visitors and
anonymous users the SQL and behaviour are unchanged.

How it works

• New generated tsvector column posts.search_response (response only,
  GIN-indexed). NULL when the post has no response, so the index stays small.
• searchPosts extends the predicate and ranking with
  q.search_response @@ ... only when user.IsCollaborator() is true.
• findSimilarPosts is unchanged.

Notes

Response search uses websearch_to_tsquery with the raw query so
version-style tokens like v2.3.1 are preserved as a single token. The
existing title/description path still uses the prefix-matching to_tsquery
flow.

No frontend changes — the search box automatically benefits from the wider
scope when used by a logged-in collaborator/administrator.