feat: Build Early Access page with backend email capture

Combined page: countdown timer (Feb 28), email capture with server
count segmentation (wired to POST /api/early-access), Founding Admin
Program (25 slots), demo dashboard preview placeholders, roadmap
voting, and launch timeline. Backend: Axum handler, migration for
early_access_signups table with email + server_count + created_at.

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

backend/src/api/mod.rs

@@ -11,3 +11,4 @@ pub mod team;
 pub mod notifications;
 pub mod license;
 pub mod store;
+pub mod early_access;