_schemas.py was a local exception to the codebase convention. The rest
of the app keeps all API request/response DTOs in decnet/web/db/models.py
alongside UserResponse, DeployIniRequest, etc. — the swarm endpoints now
follow the same convention (SwarmEnrollRequest, SwarmHostView, etc).
Deletes decnet/web/router/swarm/_schemas.py.
Splits the three grouped router files into eight api_<verb>_<resource>.py
modules under decnet/web/router/swarm/ to match the convention used by
router/fleet/ and router/config/. Shared request/response models live in
_schemas.py. Keeps each endpoint easy to locate and modify without
stepping on siblings.
