feat: persist icon and icon_color on Place entity

[MarkCesium]

Context

Persist the new icon and icon_color fields for Place. Depends on PlaceBrain/contracts#1.

Changes

• SQLAlchemy: add nullable icon: str | None and icon_color: str | None columns in backend/places/src/infra/db/models/place.py.
• DTOs: extend PlaceDTO and PlaceWithRoleDTO in backend/places/src/core/dto.py.
• Service: thread fields through create_place and update_place in backend/places/src/services/places.py. Existing role enforcement via _get_member_or_fail (OWNER / ADMIN for update) stays unchanged.
• Handler: pass through proto ↔ service in backend/places/src/handlers/places.py.
• Migration: generate via make places-migration m="add place icon fields". Never hand-write the migration file.

Acceptance

• Auto-generated Alembic migration applied cleanly on a fresh and on an existing DB.
• ruff check, ruff format --check, mypy green.
• No Kafka event schema changes.

Blocked by

• feat: add icon and icon_color fields to Place proto contracts#1 — proto fields

Blocks

• feat: expose icon and icon_color on Place HTTP schemas gateway#2 — HTTP schemas
• feat: place icons — picker UI and rendering frontend#2 — picker UI and rendering