feat: editable filename in RightSidebar (renames file on disk)

The first phase-11 file operation. Lightroom-style inline rename
of a single photo, in place, in its current directory.

Backend (PATCH /photos/{id})
- PhotoUpdate schema accepts an optional `filename`.
- When set, the handler validates: non-empty, no path separators,
  no `..`/`.`, target name doesn't already exist in the directory,
  source file exists on disk.
- os.renames the file inside its current directory, then updates
  photo.filename + photo.filepath atomically. The DB only changes
  after a successful rename — a filesystem failure leaves the
  rest of the row untouched.
- Other PhotoUpdate fields still apply afterwards in the same
  request.

Frontend (RightSidebar)
- Filename is now an editable monospace input above the Title
  input. Same draft + commit pattern as title/notes (local draft,
  resync on photo.id change, on-blur or Enter commits).
- Esc reverts to the server value.
- Client-side validation mirrors the backend (rejects path
  separators and dot-segments) and shows a toast on backend
  errors with the FastAPI detail message, then rolls the draft
  back so the input matches the still-on-disk filename.
- Removed the old read-only Filename Field from the Basic Info
  section to avoid showing the same value twice.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/schemas/photos.py

@@ -46,6 +46,7 @@ class PhotoResponse(PhotoBase):
 
 class PhotoUpdate(BaseModel):
     """Photo update schema"""
+    filename: Optional[str] = None
     user_title: Optional[str] = None
     user_notes: Optional[str] = None
     rating: Optional[int] = Field(None, ge=0, le=5)