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>

artifacts/exception_handlers.py

@@ -0,0 +1,24 @@
+from rest_framework.response import Response
+from rest_framework.views import exception_handler
+
+
+def daybook_exception_handler(exc, context):
+    """DRF exception handler that additionally maps pathological-input crashes
+    to a clean 400 instead of an unhandled 500.
+
+    Deeply nested JSON in the request body raises RecursionError inside DRF's
+    JSONParser (before any view validation runs). RecursionError subclasses
+    RuntimeError, not ValueError, so DRF's default handler doesn't map it and
+    it surfaces as a 500. Anything the default handler already knows about is
+    left to it; anything still unhandled that isn't a RecursionError is
+    returned as None so Django produces its normal 500.
+    """
+    response = exception_handler(exc, context)
+    if response is not None:
+        return response
+    if isinstance(exc, RecursionError):
+        return Response(
+            {"detail": "Request payload is nested too deeply to parse."},
+            status=400,
+        )
+    return None