feat: add landing page with beta signup + raise KB node limit to 100

Landing page at /landing with full marketing content: hero, features,
pricing, testimonials, and beta email signup form. Beta signups email
beta@resolutionflow.com via new public endpoint. Unauthenticated users
redirect to landing instead of login. Also raises KB Accelerator node
limit from 50 to 100 to accommodate dense troubleshooting articles.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/core/kb_conversion_service.py

@@ -139,7 +139,7 @@ Return a JSON object with this structure:
 4. The first node is the root of the decision tree.
 5. All `next_node_id` and option `next_node_id` references must point to existing node IDs.
 6. Detect implicit branching logic (e.g., "If X, do Y; otherwise Z") and create decision nodes.
-7. Produce at least 3 nodes. Maximum 50 nodes.
+7. Produce at least 3 nodes. Maximum 100 nodes.
 8. Use high confidence (0.9+) for directly stated steps, medium (0.7-0.89) for reasonable inferences, low (<0.7) for significant interpretation.
 9. Return ONLY valid JSON — no markdown fences, no explanation text."""