feat: expose icon and icon_color on Place HTTP schemas

[MarkCesium]

Context

Pass the new Place icon fields through from the places gRPC service to the HTTP API. Depends on PlaceBrain/contracts#1 and PlaceBrain/places#1.

Changes

• backend/gateway/src/api/places/schemas.py: add optional icon: str | None and icon_color: str | None to CreatePlaceRequest, UpdatePlaceRequest, and PlaceResponse. Update PlaceResponse.from_proto to read the new fields.
• backend/gateway/src/api/places/controllers.py: thread fields into proto messages on create and update.

Acceptance

• OpenAPI schema reflects the new fields.
• ruff, mypy green.
• Manual smoke: create a place with an icon, GET /api/places/{id} returns it.

Blocked by

• feat: add icon and icon_color fields to Place proto contracts#1 — proto fields
• feat: persist icon and icon_color on Place entity places#1 — backend persistence

Blocks

• feat: place icons — picker UI and rendering frontend#2 — picker UI and rendering