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/queries.py

@@ -49,7 +49,7 @@ def feed(
     offset: int = 0,
 ) -> list[dict]:
     """Return ranked articles, optionally filtered by topic and/or flavor."""
-    clauses = []
+    clauses = ["a.duplicate_of IS NULL"]
     params: list = []
     if accepted_only:
         clauses.append("s.accepted = 1")