Add json and csv artifact kinds with interactive viewers

Gap 1 from planning/2026-09-02-artifact_kind_gaps_for_frequent_agent_output.md.
Chose new first-class kinds over riding on html_fragment, resolving that
doc's open design question in favor of filtering support, raw-source
preservation (like markdown's .md sidecar), and server-owned validation.

- json: validated with json.loads at post time (400 on invalid JSON),
  rendered with renderjson (collapsible tree), vendored locally.
- csv: parsed server-side with the stdlib csv module into columns/rows and
  handed to Grid.js as plain data - no client-side CSV-parsing plugin
  needed, keeping normalization server-owned. Vendored locally (MIT).
- Both preserve their original text as a sidecar file, generalizing what
  was previously markdown-only handling (storage.py's SIDECAR_SUFFIXES) so
  disk-size accounting, eviction cleanup, and ?raw=1 all cover json/csv too.
- Raw JSON is embedded in a <script type="application/json"> block with
  `</script` occurrences escaped (safe because JSON's \/ escape round-trips
  correctly) - verified with a live </script>-injection payload against the
  actual running server, not just unit tests.

New migration (0012) for the KIND_CHOICES change - SQLite doesn't enforce
choices at the schema level, but Django's migration state tracks it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

artifacts/models.py

@@ -10,6 +10,8 @@ class Artifact(models.Model):
     IMAGE = "image"
     PDF = "pdf"
     BINARY = "binary"
+    JSON = "json"
+    CSV = "csv"
 
     AGENT = "agent"
     PERSON = "person"
@@ -23,6 +25,8 @@ class Artifact(models.Model):
         (IMAGE, "Image"),
         (PDF, "PDF"),
         (BINARY, "Binary"),
+        (JSON, "JSON"),
+        (CSV, "CSV"),
     ]
 
     SOURCE_TYPE_CHOICES = [