Name the sizes a node can ask for

Raw cpus and gpus are a property of the machines an installation has, so a node
written against a cluster quietly stops meaning anything when the cluster is
replaced. A node says "gpu-small" instead, and what that is stored here —
editable, and read again every time the node is built, so changing the flavor
changes what the next run gets.

Memory joins the schema properly (`ram`, in MB, accepting "2G"), along with
`duration_s` for how long a node is expected to take. That one is recorded and
shown and nothing else yet: a statement for whoever is planning around the node,
not a limit — the limit is still `timeout`.

A flavor and a number for the same thing is refused, compared by value so an
editor writing the whole object back with its defaults still round-trips. A name
nothing stores is refused at the save, which covers the canvas and `fluksio
sync` at once, and deleting one a node still asks for says which node.

Four sizes are seeded on an installation that has none, and never re-seeded:
re-adding one somebody deliberately removed is an argument nobody wins.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01A6HeySA27EkGANZN95QySW

backend/fluksio/api/main.py

@@ -5,6 +5,7 @@ from fluksio.api.routes import (
     artifacts,
     cloud,
     dashboards,
+    flavors,
     flows,
     login,
     messages,
@@ -24,6 +25,7 @@ api_router = APIRouter()
 api_router.include_router(login.router)
 api_router.include_router(users.router)
 api_router.include_router(utils.router)
+api_router.include_router(flavors.router)
 api_router.include_router(flows.router)
 api_router.include_router(flows.ws_router)
 api_router.include_router(secrets.router)