Fix #014: sanitize themed text kinds + script-blocking CSP (stored XSS)

markdown/deck/html_fragment rendered author-influenced content verbatim,
so <script>/<img onerror>/etc. executed on the Daybook origin — self-XSS
against the owner's session, and (post-sharing) unauthenticated via /s/.

Two layers, both in artifacts/views.py:
- Create-time allowlist sanitization via nh3 (maintained ammonia/Rust
  sanitizer, not deprecated bleach). Keeps formatting/tables/links/images
  (incl. relative /assets URLs) and codehilite markup; drops script, on*
  handlers, style, unknown tags, and javascript:/data: URLs. html_full is
  deliberately NOT sanitized (documented verbatim escape hatch).
- Serve-time CSP (script-src 'none') for the sanitized kinds, on both
  /a/{id} and the unauthenticated /s/{token} route. Being a response
  header, it also neutralizes scripts in artifacts stored before this fix.
  Not applied to html_full or json/csv (json/csv data is already escaped;
  their viewers need their own JS).

Verified live across all kinds; 7 regression tests added. 102 tests pass.

Not addressed (noted in the issue): CSV formula injection,
CSRF_COOKIE_HTTPONLY (moot with XSS closed), nonce CSP for json/csv/full.

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

requirements.txt

@@ -2,6 +2,7 @@ Django>=5.0,<6.0
 djangorestframework>=3.15,<4.0
 Markdown>=3.6,<4.0
 Pygments>=2.17,<3.0
+nh3>=0.2,<0.3
 openpyxl>=3.1,<4.0
 gunicorn>=23.0,<24.0
 whitenoise>=6.7,<7.0