Fix #015 (nested-JSON 500) and #016 (ragged CSV data loss)

#015 — deeply nested JSON raised RecursionError (a RuntimeError, not
ValueError) and became an unhandled 500 on two paths:
- json-kind content validation now catches RecursionError -> 400.
- Added a custom DRF EXCEPTION_HANDLER
  (artifacts/exception_handlers.py, wired via REST_FRAMEWORK) mapping
  otherwise-unhandled RecursionError -> 400, covering the parser-level
  crash on deeply nested request bodies (metadata etc.) too.

#016 — wrap_csv_viewer handed Grid.js rows without reconciling width to the
header, so over-long rows silently dropped cells. Now normalizes every row
to the max width: long rows widen the columns (auto-named "Column N"),
short rows pad with "". No cell is dropped.

Both verified live; 4 regression tests added. 106 tests pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

issues/016-csv-ragged-rows-silent-data-loss-in-render.md

@@ -1,6 +1,6 @@
 # 016 — Ragged CSV rows silently lose data in the rendered table (server doesn't normalize row width)
 
-**Status:** Open
+**Status:** Fixed
 **Severity:** Low-Medium — silent data loss in the primary rendered view;
 raw sidecar is unaffected, so it's a fidelity/display bug, not permanent
 loss.
@@ -82,4 +82,13 @@ raw sidecar.
 
 ## Resolution
 
-_(pending)_
+Implemented option (b) in `wrap_csv_viewer`: normalize every row to the
+maximum row width before handing data to Grid.js. Rows longer than the
+header widen the column set (extra columns auto-named `Column N`); rows
+shorter than the width are padded with empty strings. No cell is ever
+dropped, matching "server owns normalization, never silently lose content."
+
+Verified live with the issue's exact input (`a,b,c` / `1,2,3,4,5` / `6,7`):
+grid data becomes `columns=[a,b,c,Column 4,Column 5]`,
+`data=[[1,2,3,4,5],[6,7,"","",""]]` — the previously-dropped `4` and `5`
+now render. Test added in `tests/api/test_artifacts.py`.