Add an AI slop verdict that downrates the author's future articles

A fourth explicit verdict, "AI slop" (Vote::Slop, label `slop`), joins the
rating widget on the web, the EPUB footer links, the /r/ confirmation page,
the CLI, imports and the dashboard filters. It counts as a full negative
(curation.feedback.slop_value, -1.0) in the neighbour and affinity signals.

Beyond that, each run loads the authors whose current verdict is slop, with
no lookback, and multiplies the preliminary blend and the utility of every
candidate by that author by 1 - curation.ranking.slop_author_penalty (0.75),
so they sink before triage. The flag is recorded in signals_json, shown by
`explain` and the dashboard signals table, and the confirmation names the
author (or says no author is known, in which case only the rating applies).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VXYGBPoHZSDSfE5WcJ9bvj

src/curate/editor.rs

@@ -172,6 +172,7 @@ fn render_candidate(candidate: &Candidate) -> String {
                 "loved" => "LOVED",
                 "good" => "GOOD",
                 "not_for_me" | "down" => "NOT FOR ME",
+                "slop" => "AI SLOP",
                 other => other,
             };
             format!("{label} \"{}\" ({:.2})", neighbour.title, neighbour.cos)