feat(comments): v2 — HTMX, Turnstile, reactions, design refresh

- Extract comment templates into reusable partials (_comment.html,
  _comment_form.html, _comment_list.html, _reply_form.html, etc.)
- Add HTMX progressive enhancement: inline form submission with
  partial responses, delta polling for live updates, form reset on
  success, success/moderation toast feedback
- Integrate Cloudflare Turnstile for invisible bot protection:
  server-side token validation with hostname check, fail-closed on
  errors/timeouts, feature-flagged via TURNSTILE_SECRET_KEY env var
- Auto-approve comments that pass Turnstile; keep manual approval
  as fallback when Turnstile is disabled (model default stays False)
- Add CommentReaction model with UniqueConstraint for session-based
  anonymous reactions (heart/thumbs-up), toggle support, separate
  rate-limit bucket (20/min)
- Add comment poll endpoint (GET /comments/poll/<id>/?after_id=N)
  for HTMX delta polling without duplicates
- Update CSP middleware to allow challenges.cloudflare.com in
  script-src, connect-src, and frame-src
- Self-host htmx.min.js (v2.0.4) to minimize CSP surface area
- Add django-htmx middleware and requests to dependencies
- Add Unapprove bulk action to Wagtail admin for moderation
- Extend PII purge command to anonymize reaction session_key
- Design refresh: neon glow avatars, solid hover shadows, gradient
  section header, cyan reply borders, grid-pattern empty state,
  neon-pink focus glow on form inputs
- Add turnstile_site_key to template context via context processor
- 18 new tests covering HTMX contracts, Turnstile success/failure/
  timeout/hostname-mismatch, polling deltas, reaction toggle/dedup/
  rate-limit, CSP headers, and PII purge extension

Closes #43

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

apps/core/middleware.py

@@ -28,11 +28,12 @@ class SecurityHeadersMiddleware:
             return response
         response["Content-Security-Policy"] = (
             f"default-src 'self'; "
-            f"script-src 'self' 'nonce-{nonce}'; "
+            f"script-src 'self' 'nonce-{nonce}' https://challenges.cloudflare.com; "
             "style-src 'self' https://fonts.googleapis.com; "
             "img-src 'self' data: blob:; "
             "font-src 'self' https://fonts.gstatic.com; "
-            "connect-src 'self'; "
+            "connect-src 'self' https://challenges.cloudflare.com; "
+            "frame-src https://challenges.cloudflare.com; "
             "object-src 'none'; "
             "base-uri 'self'; "
             "frame-ancestors 'self'"