Keep the file name a node gave an artifact

A run_artifact row is keyed by the message the bytes left on, and that was
also the only name it could answer with — so an `@run:` reference resolved
through the row was the same bytes under a name its producer never chose.
The row now records the file name beside the message name; rows written
before the column answer as they always did.

The fallback also checks the bytes are still in the store, which the bare
digest spelling beside it has always done. A missing blob now fails at
submit rather than in the middle of the run that wanted it.

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

backend/fluksio/alembic/versions/e5b8c2f4a913_run_artifact_filename.py

@@ -0,0 +1,38 @@
+"""run_artifact.filename
+
+The row is keyed by the message the bytes left on, and that was also the only
+name it could answer with. A reference rebuilt from it was then the same bytes
+under a different name from the one the producing node gave them.
+
+Revision ID: e5b8c2f4a913
+Revises: a4d9e2b71c68
+Create Date: 2026-08-26
+
+"""
+
+import sqlalchemy as sa
+import sqlmodel.sql.sqltypes
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision = "e5b8c2f4a913"
+down_revision = "a4d9e2b71c68"
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    # Nullable: the rows already here never recorded one, and the message name
+    # is what they keep answering with.
+    op.add_column(
+        "run_artifact",
+        sa.Column(
+            "filename",
+            sqlmodel.sql.sqltypes.AutoString(length=255),
+            nullable=True,
+        ),
+    )
+
+
+def downgrade():
+    op.drop_column("run_artifact", "filename")