Read-time: full-article "Full story · ~N min" badge (Option B)

Replaces the gist-based read-time with the SOURCE article's full read time — the
contrast that sells the gist ("calm 1-min version here; ~10 min for the deep dive").

- goodnews/readtime.py: word_count_from_html (strips script/style/nav/header/
  footer/form/button/aside furniture before counting) + source_read_minutes
  (~225 wpm, 200-word floor, None when extraction looks failed/too thin).
- articles.source_words + read_checked_at columns (count only, never the body;
  fits the privacy posture). Idempotent migration.
- enrich.fetch_source_words + enrich_read_times: a bounded, retry-guarded cycle
  step (mirrors the image enrichers) that counts words for recent accepted
  articles. Only ever writes a real count; never overwrites good with zero. Wired
  into the cycle after recent-image enrichment.
- queries: source_words flows through _ARTICLE_COLUMNS; api exposes
  source_read_minutes on Article (null when unknown).
- home3: News card shows "Full story · ~N min", hidden entirely when null (no
  misleading "1 min").
- Tests: furniture stripping, threshold/rounding, enrich idempotency + no
  zero-overwrite, API null handling. 412 backend.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

goodnews/queries.py

@@ -55,6 +55,7 @@ _ARTICLE_COLUMNS = f"""
     s.reason_text,
     s.model_name,
     src.paywall_override AS paywall_override,
+    a.source_words,
     (SELECT group_concat(t.tag) FROM article_tags t WHERE t.article_id = a.id) AS tags,
     {RANK_SCORE_SQL} AS rank_score
 """