Media dtypes: image, audio and video as narrowed artifact references

A port may now declare `image`, `audio` or `video`. Each is the artifact
reference the engine already had, narrowed by the `media_type` on it, so a
speech recogniser declares what it eats rather than taking any bytes at all and
finding out. Bytes still never travel as a message and nothing on the wire
stops being JSON: a camera publishes one reference per frame, a microphone one
per chunk, and a reference may carry a `meta` dict nothing here interprets.

Streaming media is therefore an ordinary streaming port — with one change to
what that means. An emission used to journal an item with no payload, so
downstream read whatever was current when the item was claimed; a consumer
slower than its producer saw only the newest chunk and the ones between were
lost. That is right for a training curve and wrong for a second of speech, so
an emission now journals a `kind="emission"` item carrying its values, and the
executor hands them to the nodes reading that message instead of writing them
to state again. The value in state stays the latest, which is what everything
else reads, and the wave is filtered by what actually changed rather than
walking everything reachable. No queue serialization change — the existing
`outputs` field carries it.

Continuous media makes the store's missing GC a real problem, so this closes
it: `sweep_artifacts` runs hourly, keeps every digest a `run_artifact` row
records or a live message holds, spares anything written in the last hour, and
stands aside entirely while a run is in flight, since a node may store a
checkpoint long before it returns the reference to it. That also collects the
orphans a deleted flow has always left behind. `ARTIFACT_GC_INTERVAL_S=0` turns
it off.

Around the edges: `GET /artifacts/{digest}` serves the media type the caller
passes and answers ranged requests, so a browser plays a clip rather than
downloading it; `PUT` spools to disk instead of holding the whole body in
memory, as does `save_artifact` given a path; a Media widget draws whatever its
message points at, and a wall panel may fetch the bytes its own tiles are
showing and nothing else; and a connector gets `save_artifact`, for a device
whose readings are bytes.

What this cannot do is live video: a frame every second or two is a glance, and
the honest answer above that is the camera's own stream, which the widget takes
as a URL and the browser plays from source.

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

backend/fluksio/flow/connector.py

@@ -17,7 +17,10 @@ What a connector gets from the base class:
   be known by the engine;
 * :meth:`ConnectorNode.write`, the other direction — values arriving on the
   node's input ports, for a connector that commands something rather than only
-  reading it.
+  reading it;
+* :meth:`ConnectorNode.save_artifact`, for a device whose readings are bytes —
+  a camera frame, a recorded clip — which travel as a reference on a media
+  port rather than as the message itself.
 
 The message schemas and the parameter model are the rest of the contract, and
 they are the same ones the built-in nodes use. See ``docs/connectors/`` for the
@@ -37,6 +40,8 @@ from fluksio.flow.nodes import Node
 if TYPE_CHECKING:
     from fastapi import FastAPI
 
+    from fluksio.flow.artifacts import ArtifactStore
+
 logger = logging.getLogger(__name__)
 
 #: Bumped when a change would break connectors written against the old surface.
@@ -81,7 +86,7 @@ class ConnectorNode(Node):
             description="Seconds between polls; 0 polls never.",
         )
 
-    __slots__ = ("config", "_poll_task", "_stop_event", "_last_published")
+    __slots__ = ("config", "_poll_task", "_stop_event", "_last_published", "_artifacts")
 
     def __init__(self, **kwargs: Any) -> None:
         super().__init__(f=self._dispatch, **kwargs)
@@ -89,6 +94,7 @@ class ConnectorNode(Node):
         self._poll_task: asyncio.Task[None] | None = None
         self._stop_event: asyncio.Event | None = None
         self._last_published: dict[str, Any] = {}
+        self._artifacts: ArtifactStore | None = None
 
     def _dispatch(self, params: dict[str, Any], **ports: Any) -> dict[str, Any] | None:
         """The scheduler's entry point. Settings are already on ``self.config``."""
@@ -122,7 +128,30 @@ class ConnectorNode(Node):
     # What the engine drives
     # -------------------------------------------------------------------------
 
+    def save_artifact(
+        self,
+        data: bytes,
+        name: str = "",
+        media_type: str = "application/octet-stream",
+    ) -> dict[str, Any]:
+        """Store bytes and return the reference to publish on a media port.
+
+        A camera frame or a recorded clip is far too big to be a message, so a
+        connector publishing one publishes this instead: the bytes go to the
+        store and the reference names them, which is what an ``image``,
+        ``audio`` or ``video`` port carries.
+
+        Only available once the node has started — the store belongs to the
+        engine, and is handed over then.
+        """
+        if self._artifacts is None:
+            raise RuntimeError(
+                "no artifact store: a connector can only save bytes once it has started"
+            )
+        return self._artifacts.put([data], name=name, media_type=media_type)
+
     async def start(self, app: FastAPI | None = None) -> None:
+        self._artifacts = getattr(app.state, "artifact_store", None) if app else None
         if self.config.poll_interval > 0 and self._stop_event is None:
             self._stop_event = asyncio.Event()
             self._poll_task = self._run_supervised("poll", self._poll_loop)