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/internal/shared/objectstorage/client.go

@@ -12,6 +12,7 @@ import (
 )
 
 var ErrNotConfigured = errors.New("object storage is not configured")
+var ErrObjectNotFound = errors.New("object storage object not found")
 
 type Client interface {
 	Validate() error