feat(stories): per-clip volume control on the timeline

Each story item now carries a volume column (linear gain, default 1.0,
clamped 0.0–2.0 server-side). New PUT /stories/{}/items/{}/volume route
+ useUpdateStoryItemVolume hook + a Volume2 icon in the clip-edit
toolbar that opens a popover with a 0–200% slider. Local slider state
drives the visual during a drag; the persist fires once on
onValueCommit, mirroring the generation-page slider pattern.

Web Audio playback inserts a per-clip GainNode between source and
master so volume changes apply live without re-decoding the buffer
(source -> clipGain -> masterGain -> destination). Server-side
mixdown in export multiplies the trimmed clip by its volume before
summing into the timeline. Split + duplicate carry the volume forward
to the new clips so trimming a faded section keeps the level you set.

Migration adds the volume column with default 1.0 so existing rows
read as full volume.

backend/routes/stories.py

@@ -151,6 +151,20 @@ async def trim_story_item(
     return item
 
 
+@router.put("/stories/{story_id}/items/{item_id}/volume", response_model=models.StoryItemDetail)
+async def update_story_item_volume(
+    story_id: str,
+    item_id: str,
+    data: models.StoryItemVolumeUpdate,
+    db: Session = Depends(get_db),
+):
+    """Set a story item's per-clip volume (linear gain, 0.0–2.0)."""
+    item = await stories.update_story_item_volume(story_id, item_id, data, db)
+    if item is None:
+        raise HTTPException(status_code=404, detail="Story item not found")
+    return item
+
+
 @router.post("/stories/{story_id}/items/{item_id}/split", response_model=list[models.StoryItemDetail])
 async def split_story_item(
     story_id: str,