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/routes/flows.py

@@ -57,7 +57,7 @@ from fluksio.flow.store import (
     LibNotFound,
     StaleVersion,
 )
-from fluksio.models import Message, Run, RunArtifact, RunMetric, RunNode
+from fluksio.models import Flavor, Message, Run, RunArtifact, RunMetric, RunNode
 
 router = APIRouter(
     prefix="/flows", tags=["flows"], dependencies=[Depends(get_current_user)]
@@ -368,6 +368,31 @@ def read_flow(name: str, controller: FlowControllerDep) -> Any:
     return _detail(controller, _read_flow(controller, name))
 
 
+def _check_flavors(definition: FlowDef) -> None:
+    """Refuse a size nobody stored, here rather than when the node is built.
+
+    Catching it at the save covers the canvas and ``fluksio sync`` in one
+    place; a node that only finds out at build time is a red node somebody has
+    to go and look at.
+    """
+    named = {
+        node.resources.flavor
+        for node in definition.nodes
+        if node.resources is not None and node.resources.flavor
+    }
+    if not named:
+        return
+    with Session(engine) as session:
+        known = set(session.exec(select(Flavor.name)).all())
+    unknown = sorted(named - known)
+    if unknown:
+        raise HTTPException(
+            status_code=422,
+            detail=f"No flavor named '{unknown[0]}' "
+            f"(known: {', '.join(sorted(known)) or 'none'})",
+        )
+
+
 @router.put("/{name}", response_model=FlowDetail)
 async def save_flow(
     name: str,
@@ -390,6 +415,8 @@ async def save_flow(
     if len(duplicates) != len(definition.nodes):
         raise HTTPException(status_code=400, detail="Node names must be unique")
 
+    _check_flavors(definition)
+
     try:
         stored = await run_in_threadpool(
             controller.store.write_draft, definition, definition.version