feat(stories): import external audio into the timeline (drag-drop + picker)

You can now drop a music file onto the story content area or pick one through the new "Import audio" button in the add-clip popover. Both call POST /generate/import which writes the file to data/generations/<id>.<ext>, probes duration via librosa, and inserts a Generation row pointing at a singleton "Imported Audio" profile (created lazily on first import). The existing addStoryItem flow takes over from there — the timeline doesn't care that the row didn't come out of TTS.

Engine field on the row is "import"; it's surfaced on StoryItemDetail so the chat list shows a music icon instead of the (missing) profile avatar and both the dropdown and the track-editor toolbar hide the Regenerate action — there's nothing to regenerate. Accepted formats: wav/mp3/flac/ogg/m4a/aac/webm, capped at 200 MB. Translation keys added across en/ja/zh-CN/zh-TW.

backend/models.py

@@ -597,6 +597,7 @@ class StoryItemDetail(BaseModel):
     duration: float
     seed: Optional[int]
     instruct: Optional[str]
+    engine: Optional[str] = None
     generation_created_at: datetime
     # Versions available for this generation
     versions: Optional[List["GenerationVersionResponse"]] = None