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>

static/vendor/gridjs/LICENSE

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) Afshin Mehrabani <afshin.meh@gmail.com>
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.