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/core/db.py

@@ -20,7 +20,7 @@ from sqlmodel import Session, SQLModel, create_engine, select
 
 from fluksio import crud
 from fluksio.core.config import settings
-from fluksio.models import User, UserCreate
+from fluksio.models import Flavor, User, UserCreate
 
 logger = logging.getLogger(__name__)
 
@@ -110,8 +110,40 @@ def init_db(session: Session) -> None:
         user = crud.create_user(session=session, user_create=user_in)
 
 
+#: The sizes an installation starts with. Written once, when there are none,
+#: and editable from there — what a name means is a property of the machines
+#: this installation has, and nothing here knows what those are.
+SEED_FLAVORS: tuple[dict[str, Any], ...] = (
+    {"name": "small", "cpus": 1, "ram": 2048, "description": "A poll, a threshold"},
+    {"name": "medium", "cpus": 4, "ram": 8192, "description": "A step that computes"},
+    {"name": "large", "cpus": 8, "ram": 16384, "description": "A heavy step"},
+    {
+        "name": "gpu-small",
+        "cpus": 4,
+        "gpus": 1,
+        "ram": 16384,
+        "description": "One card, and cores to feed it",
+    },
+)
+
+
+def seed_flavors(session: Session) -> None:
+    """Give a new installation sizes to pick from, once.
+
+    Only when there are none at all: they are editable, and re-adding one that
+    somebody deliberately removed would be an argument nobody can win.
+    """
+    if session.exec(select(Flavor)).first() is not None:
+        return
+    for row in SEED_FLAVORS:
+        session.add(Flavor(**row))
+    session.commit()
+    logger.info("seeded %d resource flavors", len(SEED_FLAVORS))
+
+
 def prepare(engine: Engine) -> None:
     """Everything that has to be true before the app serves a request."""
     migrate(engine)
     with Session(engine) as session:
         init_db(session)
+        seed_flavors(session)