fix(schema): allow empty files[] in source_media_review artifact

review_source_media deliberately returns files:[] with a 'no source media —
fully generated production' summary when no user media is supplied or none can
be reviewed, but the schema declared files.minItems:1, so that intended
artifact failed its own validation. Relax files.minItems to 0 to match the
code's deliberate empty-media state (planning_implications still carries an
entry, so its minItems:1 remains satisfied).

Adds a regression test validating the no-source-media artifact.

Closes #269

tests/lib/test_source_media_review_empty.py

@@ -0,0 +1,29 @@
+"""Regression test for source_media_review empty-files artifact validity.
+
+review_source_media deliberately returns an artifact with files:[] when no user
+media was supplied (or none could be reviewed) — a valid "fully generated
+production" state. The schema declared files.minItems: 1, so that intended
+artifact failed its own schema validation.
+"""
+
+import sys
+from pathlib import Path
+
+PROJECT_ROOT = Path(__file__).resolve().parent.parent.parent
+sys.path.insert(0, str(PROJECT_ROOT))
+
+from lib.source_media_review import review_source_media  # noqa: E402
+from schemas.artifacts import validate_artifact  # noqa: E402
+
+
+def test_no_source_media_produces_schema_valid_artifact(tmp_path):
+    art = review_source_media([tmp_path / "does-not-exist.mp4"], {})
+    assert art["files"] == []
+    # Must not raise — this is a legitimate no-source-media artifact.
+    validate_artifact("source_media_review", art)
+
+
+def test_no_files_at_all_is_schema_valid():
+    art = review_source_media([], {})
+    assert art["files"] == []
+    validate_artifact("source_media_review", art)