Backfill pattern analysis flag on existing banners

Set pattern_analysis_requested_at on all existing banners so the worker
runs once per banner on release and merges any existing prefix groups.

Signed-off-by: Émile Ré <emile@getprobo.com>

pkg/coredata/migrations/20260429T102803Z.sql

@@ -14,3 +14,5 @@
 
 ALTER TABLE cookie_banners
     ADD COLUMN pattern_analysis_requested_at TIMESTAMP WITH TIME ZONE;
+
+UPDATE cookie_banners SET pattern_analysis_requested_at = NOW();