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/database/models.py

@@ -105,6 +105,7 @@ class StoryItem(Base):
     track = Column(Integer, nullable=False, default=0)
     trim_start_ms = Column(Integer, nullable=False, default=0)
     trim_end_ms = Column(Integer, nullable=False, default=0)
+    volume = Column(Float, nullable=False, default=1.0)
     created_at = Column(DateTime, default=datetime.utcnow)