Partial index for claims still awaiting topic tagging

Each enrich.topics batch scanned all claims for untagged ones; after the
history backfill a run took hours and blocked the enrich queue.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/migrations/0006_untagged_claims_index.sql

@@ -0,0 +1,7 @@
+-- enrich.topics fetches the next batch of untagged claims on every iteration; without an index each batch scanned
+-- all claims (over a million after the history backfill) and a run took hours, blocking the enrich queue. The
+-- predicate must stay identical to the query in enrich/topics.py. Built CONCURRENTLY by hand on 2026-11-14.
+CREATE INDEX IF NOT EXISTS claim_untagged_idx ON claim (id)
+    WHERE claim_type IN ('media_mention', 'statement', 'donation', 'side_job', 'appearance', 'meeting', 'travel',
+                         'money_flow', 'government_decision', 'wiki_edit', 'career')
+      AND NOT (data ? 'topics_tagged');