Fix artifact dict key mismatch (hash -> cid)

Template runs/detail.html expects artifact.cid but code provided
artifact.hash, causing UndefinedError when viewing run details.

- Change run_service.get_run_artifacts to return 'cid' key
- Change runs.py router inline artifact creation to use 'cid' key
- Add regression tests for artifact data structure

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/routers/runs.py

@@ -258,7 +258,7 @@ async def get_run(
             except Exception:
                 pass
             artifacts.append({
-                "hash": output_cid,
+                "cid": output_cid,
                 "step_name": "Output",
                 "media_type": media_type or "application/octet-stream",
             })

app/services/run_service.py

@@ -547,7 +547,7 @@ class RunService:
                 path = self.cache.get_by_cid(cid)
                 if path and path.exists():
                     return {
-                        "hash": cid,
+                        "cid": cid,
                         "size_bytes": path.stat().st_size,
                         "media_type": detect_media_type(path),
                         "role": role,