fix: disk stats show LUKS data partition, not 29GB root

system.stats (Home page) and monitoring collector both used df /
which shows the small 29GB root partition. Now prefers
/var/lib/archipelago (the LUKS encrypted data partition) when it
exists — showing the actual 1.8TB storage users care about.

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

core/archipelago/src/api/rpc/system/handlers.rs

@@ -54,7 +54,10 @@ impl RpcHandler {
         let load = read_loadavg().await.unwrap_or((0.0, 0.0, 0.0));
         let cpu = read_cpu_usage().await.unwrap_or(0.0);
         let (mem_used, mem_total) = read_meminfo().await.unwrap_or((0, 0));
-        let (disk_used, disk_total) = read_disk_usage().await.unwrap_or((0, 0));
+        // Prefer encrypted data partition if it exists
+        let data_path = std::path::Path::new("/var/lib/archipelago");
+        let df_target = if data_path.exists() { "/var/lib/archipelago" } else { "/" };
+        let (disk_used, disk_total) = read_disk_usage_path(df_target).await.unwrap_or((0, 0));
 
         Ok(serde_json::json!({
             "uptime_secs": uptime as u64,

core/archipelago/src/monitoring/collector.rs

@@ -113,10 +113,15 @@ fn parse_kb(val: &str) -> Result<u64> {
         .context("parse meminfo kB value")
 }
 
-/// Read disk used/total via `df` for the root filesystem.
+/// Read disk used/total via `df`, preferring the encrypted data partition.
 async fn read_disk_usage() -> Result<(u64, u64)> {
+    let target = if std::path::Path::new("/var/lib/archipelago").exists() {
+        "/var/lib/archipelago"
+    } else {
+        "/"
+    };
     let output = tokio::process::Command::new("df")
-        .args(["--block-size=1", "--output=used,size", "/"])
+        .args(["--block-size=1", "--output=used,size", target])
         .output()
         .await
         .context("Failed to run df")?;