feat: add content media dam uploads

docs/TASKS/content/008-multi-channel-preview-editor.md

@@ -26,13 +26,16 @@ The editor should use one shared content body for every selected target channel,
 - Keep target copy/title synchronized by default, with per-target opt-out.
 - Show title editing only for networks that normally need titles, such as YouTube, Reddit, Website, and newsletters.
 - Let each target choose its own media requirement.
-- Preserve existing save payloads and backend contracts.
+- Let authors upload media from the editor; uploaded media is stored as a DAM asset.
+- Let authors attach an existing workspace DAM asset to the active target preview.
+- Preserve existing content item save payloads.
 
 ## Validation
 
 ```bash
 cd frontend
 npm run build
+dotnet build backend/Socialize.slnx
 ```
 
 ## Acceptance Criteria
@@ -52,3 +55,5 @@ npm run build
 - [x] Channel copy/title is synchronized by default and can be unsynchronized per target.
 - [x] Title input only appears for title-oriented targets.
 - [x] Each target can choose no media, image, video, clip, or carousel independently.
+- [x] Authors can upload media from a target preview and create a DAM asset.
+- [x] Authors can choose an existing workspace DAM asset for a target preview.