feat(kol): store KOL prompt content in database with object storage fallback

Add prompt_content columns to kol_prompts and kol_prompt_revisions so
prompt bodies live alongside their metadata, eliminating an extra round
trip to object storage for the common read path while keeping the old
asset key as a fallback for legacy rows.

Reads go through a singleflight-deduped, cache-backed loader that
prefers the database column, falls back to the asset key, and tolerates
missing objects via a new objectstorage.ErrObjectNotFound returned by
both the Aliyun OSS and MinIO clients on 404/NoSuchKey responses.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/migrations/20260524103000_store_kol_prompt_content_in_db.down.sql

@@ -0,0 +1,5 @@
+ALTER TABLE kol_prompt_revisions
+    DROP COLUMN IF EXISTS prompt_content;
+
+ALTER TABLE kol_prompts
+    DROP COLUMN IF EXISTS prompt_content;

server/migrations/20260524103000_store_kol_prompt_content_in_db.up.sql

@@ -0,0 +1,5 @@
+ALTER TABLE kol_prompts
+    ADD COLUMN IF NOT EXISTS prompt_content TEXT;
+
+ALTER TABLE kol_prompt_revisions
+    ADD COLUMN IF NOT EXISTS prompt_content TEXT;