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/models.py

@@ -598,6 +598,7 @@ class StoryItemDetail(BaseModel):
     seed: Optional[int]
     instruct: Optional[str]
     engine: Optional[str] = None
+    volume: float = 1.0
     generation_created_at: datetime
     # Versions available for this generation
     versions: Optional[List["GenerationVersionResponse"]] = None
@@ -674,6 +675,17 @@ class StoryItemVersionUpdate(BaseModel):
     version_id: Optional[str] = None  # null = use generation default
 
 
+class StoryItemVolumeUpdate(BaseModel):
+    """Request model for adjusting a story item's playback volume.
+
+    Linear gain. ``1.0`` is the original level, ``0.0`` is silent. Capped
+    above 1.0 so a too-aggressive boost can't blow out the mix or clip
+    the export.
+    """
+
+    volume: float = Field(..., ge=0.0, le=2.0)
+
+
 class EffectConfig(BaseModel):
     """A single effect in an effects chain."""