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/db.py

@@ -49,6 +49,8 @@ CREATE TABLE IF NOT EXISTS articles (
     title_hash TEXT,
     duplicate_of INTEGER REFERENCES articles(id) ON DELETE SET NULL,
     image_checked_at TEXT,
+    source_words INTEGER,          -- full-article word count (metadata only; never the body)
+    read_checked_at TEXT,          -- when we last tried to count words (retry guard)
     FOREIGN KEY (source_id) REFERENCES sources(id)
 );
 
@@ -595,6 +597,10 @@ def _migrate(conn: sqlite3.Connection) -> None:
         )
     if "image_checked_at" not in article_cols:
         conn.execute("ALTER TABLE articles ADD COLUMN image_checked_at TEXT")
+    if "source_words" not in article_cols:        # full-article read-time (count only, no body)
+        conn.execute("ALTER TABLE articles ADD COLUMN source_words INTEGER")
+    if "read_checked_at" not in article_cols:
+        conn.execute("ALTER TABLE articles ADD COLUMN read_checked_at TEXT")
     # Created here (not in SCHEMA) so it runs after the column exists on upgrades.
     conn.execute("CREATE INDEX IF NOT EXISTS idx_articles_duplicate_of ON articles(duplicate_of)")