feat(marker): add optional color field to marker data model

Add a user-chosen hex color (e.g. '#ff5722') to each marker, stored in
Link._markers alongside bpf/tag/enabled. The color:

- Is serialized with the link (asdict) and persisted in the topology, so
  it survives project reload and is consistent across devices.
- Never reaches uBridge (compute ignores it) — it is purely a Web UI
  concern.
- Can be updated independently of the BPF (color-only changes skip the
  ubridge round-trip).

MarkerCreate schema gains color: Optional[str]; start_marker and
update_marker carry it through; REST create/update endpoints pass it.

The marker.match WebSocket event stays lean (no color) — the Web UI maps
filter -> color from the link state it already holds.

gns3server/schemas/controller/links.py

@@ -153,6 +153,10 @@ class MarkerCreate(BaseModel):
     bpf: str
     tag: Optional[int] = None
     link_id: Optional[str] = None
+    color: Optional[str] = Field(
+        None,
+        description="User-chosen hex color for this marker in the Web UI, e.g. '#ff5722'"
+    )
 
 
 class MarkerDelete(BaseModel):