fix(media): prevent attaching an asset to a target twice

Drop `position` from the `asset_usages` unique constraint so the same
asset can't be attached to one target/slot more than once, and update
the insert's onConflict target + lookup to match the narrower key.

Guard it at the service layer too: attachAssetService now rejects an
attach when the asset already has a usage for that target with a new
ASSET_ALREADY_ATTACHED (409) error, backed by an
assetUsageExistsForTarget query.

packages/core/src/services/assets/assets.ts

@@ -17,6 +17,7 @@ import type {
 } from '@cio/utils/validation/assets';
 import type { TTranscriptResponse, TUpdateTranscript } from '@cio/utils/validation/media';
 import {
+  assetUsageExistsForTarget,
   createAssetUsage,
   createHlsAssetPlaceholder,
   createOrGetAssetByStorageKey,
@@ -503,6 +504,12 @@ export async function getAssetUsageGraphService(orgId: string, assetId: string)
 export async function attachAssetService(orgId: string, assetId: string, profileId: string, input: TAssetAttach) {
   try {
     await getAssetService(orgId, assetId);
+
+    const alreadyAttached = await assetUsageExistsForTarget(assetId, orgId, input.targetType, input.targetId);
+    if (alreadyAttached) {
+      throw new AppError('Asset is already attached to this target', ErrorCodes.ASSET_ALREADY_ATTACHED, 409);
+    }
+
     return await createAssetUsage({
       organizationId: orgId,
       assetId,