feat(copilot): finish configuration center

backend/migrations/000056_make_article_embedding_dimension_dynamic.up.sql

@@ -0,0 +1,7 @@
+-- Copilot embedding dimensions are configured at runtime. Remove the fixed
+-- vector(1536) typmod so rebuilt article embeddings can use the selected model.
+DROP INDEX IF EXISTS idx_article_embeddings_vector;
+
+ALTER TABLE article_embeddings
+  ALTER COLUMN vector_embedding TYPE vector
+  USING vector_embedding::vector;