openai/gpt-chat-latest → B (2:3)

Side B centralizes path normalization, canonicalization, and room-aware URL generation into a shared `slug_types::paths` module, replacing duplicated string helpers with strongly typed `GardenItemUrl`, `ForumThreadUrl`, and related types across server and API code. This is a broad architectural improvement that reduces duplication and makes path handling consistent across serialization and validation, whereas Side A is a more localized storage refactor from `Deque` to append-only `List` with read-time capping and accompanying tests.