Add semantic cross-source dedup via local embeddings

- LocalModelClient.embed() calls the OpenAI-compatible /embeddings endpoint
  (local nomic model); base_url shared with chat, model via GOODNEWS_EMBED_MODEL.
- New article_embeddings table and articles.duplicate_of column (+ migration).
- dedup module: embeds missing articles, clusters near-identical stories within
  a date window by cosine similarity (pure-stdlib, vectors normalised once), and
  marks all but the highest-ranked member of each cluster as a duplicate.
- 'dedup' CLI command; cycle now runs poll -> classify -> dedup -> brief.
- Feed and brief queries hide duplicates, so a story carried by multiple
  outlets shows once.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

goodnews/briefs.py

@@ -118,6 +118,7 @@ def _candidate_articles(
         JOIN sources src ON src.id = a.source_id
         JOIN article_scores s ON s.article_id = a.id
         WHERE s.accepted = 1
+          AND a.duplicate_of IS NULL
           AND date(COALESCE(a.published_at, a.discovered_at)) <= date(?)
           AND date(COALESCE(a.published_at, a.discovered_at)) > date(?, '-' || ? || ' days')
           AND a.id NOT IN (