Stage caching for batch runs, and an engine that lives in the command

A code node in a batch run is now fingerprinted by its source, its raw
settings and the values it reads — an artifact input counting as its digest,
which is what the content addressing was always for. A run that finds the key
restores what the earlier one returned and skips the node, recorded as
`cached`. The run history is the cache: `run_node.outputs` beside the
`cache_key` the schema already had, no second store. On for code nodes, never
for the built-in and connector types that have side effects; off per node with
`@node(cache=False)` and per run with `--no-cache`.

Emissions are not replayed on a hit, so a cached training node returns its
result without redrawing its curve. Recorded in NOTEPAD.md with the two other
deliberate limits.

`fluksio run --local` boots the real app in the command's own process and
drives it through its ASGI interface behind the ordinary client, so a run no
longer needs a `serve` terminal beside it — same data directory, same history,
and the cache carries between the two. It always waits, because the engine it
starts lives exactly as long as the command.

Also: `fluksio sweep --param lr=0.1,0.01` for the product of the lists,
`run --follow` for a run's numbers as they arrive, Ctrl-C cancelling a waited
run rather than abandoning it, coloured statuses on a terminal, and `name`
made optional on the metrics endpoint so a follower can ask for every series.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/fluksio/models.py

@@ -342,6 +342,8 @@ class Run(SQLModel, table=True):
     parent_id: str | None = Field(default=None, max_length=64)
     #: api, hook, sweep or cli.
     cause: str = Field(default="api", max_length=32)
+    #: Re-execute every node, whatever the stage cache holds for it.
+    no_cache: bool = False
     #: queued, running, ok, error, cancelled or abandoned.
     status: str = Field(default="queued", index=True, max_length=16)
     #: Why it is where it is: what it waits for, or what went wrong.
@@ -392,10 +394,14 @@ class RunNode(SQLModel, table=True):
     worker: str = Field(default="local", max_length=64)
     error: str = ""
     logs: str = ""
-    #: Everything this node's output depends on, hashed. Recorded from the
-    #: start so that skipping a stage whose inputs have not changed is later a
-    #: lookup rather than a migration.
+    #: Everything this node's output depends on, hashed: its source, its
+    #: settings and the values it read. What a later run looks itself up by.
     cache_key: str = Field(default="", index=True, max_length=64)
+    #: What it returned, as canonical JSON, so a node with this key can be
+    #: skipped and its outputs restored. None when it may not be reused —
+    #: opted out, too large, or a value JSON cannot carry. Written with
+    #: `cache_key` or not at all, so a keyed row is always restorable.
+    outputs: str | None = None
 
 
 class RunMetric(SQLModel, table=True):