Add version control to track editor, restyle story list

- Story items can be pinned to a specific generation version via
  toolbar dropdown (shows when clip is selected and has >1 version)
- version_id column on story_items with migration, validated against
  the generation's versions before saving
- Split/duplicate preserve the source clip's pinned version
- Export and playback resolve version-specific audio paths
- Extracted _build_item_detail helper in stories.py (DRY cleanup)
- Story list restyled from rounded cards to flat rows with rounded
  hover/active states, gradient header fade, and dynamic bottom
  padding that accounts for track editor + generate box

backend/models.py

@@ -276,6 +276,7 @@ class StoryItemDetail(BaseModel):
     id: str
     story_id: str
     generation_id: str
+    version_id: Optional[str] = None
     start_time_ms: int
     track: int = 0
     trim_start_ms: int = 0
@@ -291,6 +292,9 @@ class StoryItemDetail(BaseModel):
     seed: Optional[int]
     instruct: Optional[str]
     generation_created_at: datetime
+    # Versions available for this generation
+    versions: Optional[List["GenerationVersionResponse"]] = None
+    active_version_id: Optional[str] = None
 
     class Config:
         from_attributes = True
@@ -349,6 +353,11 @@ class StoryItemSplit(BaseModel):
     split_time_ms: int = Field(..., ge=0)  # Time within the clip to split at (relative to clip start)
 
 
+class StoryItemVersionUpdate(BaseModel):
+    """Request model for setting a story item's pinned version."""
+    version_id: Optional[str] = None  # null = use generation default
+
+
 # ============================================
 # Effects & Versions
 # ============================================