fix: add health RPC handler, Nostr connect timeouts, atomic backup restore, nginx rate limits

- R1: Add health RPC endpoint with crash recovery status, uptime, and version
- R2: Wrap all 5 Nostr client.connect() calls in 10s timeout
- R3: Make backup restore atomic with staging dir and rollback on failure
- I1: Add rate limiting, body size, and proxy timeouts to unauthenticated nginx endpoints

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

core/archipelago/src/main.rs

@@ -53,6 +53,7 @@ const DEV_DEFAULT_PASSWORD: &str = "password123";
 #[tokio::main]
 async fn main() -> Result<()> {
     let startup_start = std::time::Instant::now();
+    crash_recovery::init_start_time();
 
     // Initialize tracing
     tracing_subscriber::fmt()