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/main.rs

@@ -88,6 +88,7 @@ async fn main() -> anyhow::Result<()> {
         .nest("/api/notifications", api::notifications::router())
         .nest("/api/license", api::license::router())
         .nest("/api/store", api::store::router())
+        .nest("/api/early-access", api::early_access::router())
         .layer(cors)
         .layer(TraceLayer::new_for_http())
         .with_state(state);