feat(kol): add kol_prompt_id column to articles

server/migrations/20260417100100_alter_articles_for_kol.down.sql

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+DROP INDEX IF EXISTS idx_article_kol_prompt;
+ALTER TABLE articles DROP COLUMN IF EXISTS kol_prompt_id;

server/migrations/20260417100100_alter_articles_for_kol.up.sql

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+ALTER TABLE articles
+    ADD COLUMN kol_prompt_id BIGINT REFERENCES kol_prompts(id);
+
+CREATE INDEX idx_article_kol_prompt
+    ON articles(kol_prompt_id)
+    WHERE kol_prompt_id IS NOT NULL AND deleted_at IS NULL;