feat(imitation): accept pasted source content as an alternative to URL

- Make source_url optional and add source_content to the request; the
  two are normalized into a source kind ("url" | "content"), URL wins
  when both are present, and missing both returns source_required
- Skip web fetching in the generation worker when pasted content is
  provided; truncate it to the same 24,000-rune cap
- Label untitled pasted-content jobs with a locale-aware display title
  and mark the source kind in wizard state and prompt metadata
- Cover source normalization, truncation, and display title with tests

packages/shared-types/src/index.ts

@@ -1105,7 +1105,8 @@ export interface CreateArticleRequest {
 }
 
 export interface GenerateImitationRequest {
-  source_url: string
+  source_url?: string
+  source_content?: string
   source_title?: string
   locale?: string
   brand_name: string