feat: add icon and icon_color fields to Place proto

[MarkCesium]

Context

Place currently has no visual identity beyond its name. On the frontend this is filled by a deterministic colored-initial placeholder (PlaceAvatar in the frontend repo). We want users to pick a curated icon (from a Lucide preset) and a palette color per place, persisted server-side so the choice follows the user across devices.

This issue covers only the contracts side. Backend and frontend changes are tracked in sibling issues.

Changes

Extend contracts/proto/places.proto with two optional string fields on:

• CreatePlaceRequest
• GetPlaceResponse
• PlaceSummary
• UpdatePlaceRequest

Fields:

• icon — curated key string (e.g. home, building, warehouse). Kept as an opaque string for forward-compatibility; the frontend maps it to a Lucide component. Do not hardcode an enum server-side.
• icon_color — palette key string (e.g. blue, green). Never a hex — palette is defined in the frontend CSS variables for proper light/dark theme support.

Bump contracts/pyproject.toml to the next minor version and push. Follow the standard contracts update workflow from the root CLAUDE.md.

Acceptance

• Proto updated, version bumped, CI publishes to PyPI.
• No Kafka event changes: icons are UI metadata; consumers (devices, collector) do not need them.

Blocks

• feat: persist icon and icon_color on Place entity places#1 — backend persistence
• 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