Fetch a run's artifacts from the CLI

`save_artifact` had no download counterpart: the run detail listed a run's
files and nothing in `fluksio --help` fetched one. `fluksio artifacts RUN`
lists them, `fluksio artifacts RUN NAME` writes one — under the name the
node saved it as, since the message name is chosen for the graph. The run
detail now carries that filename, which it held in the table and did not
report.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019Hra4ndWMCLU5F3KjUuVAc

backend/fluksio/api/routes/runs.py

@@ -104,6 +104,10 @@ class ArtifactRow(BaseModel):
     digest: str
     size: int
     media_type: str
+    #: What it was called where it was written, so something downloading it can
+    #: give it that name back rather than the message's. Absent when the node
+    #: never said one.
+    filename: str | None = None
 
 
 class RunRow(BaseModel):