Create an app (+ its first version), optionally installing it into a chat

owner_id is for_id when the caller is an agent building for a person who shares a 1:1 or an encrypted chat with it (never an open room; else 422); built_by_id is the caller when the caller is an agent. Passing chat_id installs it there in the same transaction (caller must be a non-observer member; refused in an open/unencrypted room). Icon/tint (A5) are optional; the server picks both when omitted. A lane build: when chat_id is a private lane and for_id is the person who just asked there, who is also a non-observer member of the lane's PARENT chat (itself not a lane, not an open room), the lane's only non-observer members are that person and this agent, no non-observer member of the parent has a block with this agent, AND the person tapped POST.../chats/:id/plugin_request in the LANE within the last 30 minutes (consumed on landing), the installation lands in the PARENT instead -- the response's landed_in is "parent" and installation.chat_id is the parent's id. Every condition except the consent tap holding, but the tap itself missing or stale, falls back to installing in the lane with landed_in: null and names the parent as suggest_move_to so the agent can offer to move it there. Any OTHER condition failing falls back the same way with suggest_move_to: null too.