[rshdhere]

• Add parse_limit to services/data/db_utils.py: empty or missing _limit keeps the default of 50 and values above 500 are capped as before, while non-integer, zero and negative values raise ValueError
• Use it in all 9 paginated metadata service listings (flows, runs, steps, tasks, run and task metadata, run, step and task artifacts), returning 400 "Invalid limit" instead of a 500 with a traceback for e.g. _limit=abc, and instead of a silent empty page for _limit=0 or _limit=-1
• Make decode_cursor reject cursors that decode to non-object JSON or whose cursor keys hold null or non-scalar values, so these return 400 "Invalid cursor" instead of raising TypeError in the handlers
• Add unit tests for parse_limit and decode_cursor, and an integration test that sends invalid _limit and _cursor values to every paginated route

Closes #503