Fleet P1: BYO Tailscale + network metrics, Tailscale SSH, exec hardening

Security hardening:
- The gateway no longer sends arbitrary shell to nodes. The WSS exec op is
  replaced by a typed `verify` op the daemon runs itself (fixed host+docker
  check); future container ops are typed too. cm-api NodeHub.verify() + the
  daemon's handle_command only dispatches vetted ops.

BYO Tailscale:
- migrations/0019_workspace_tailscale.sql + cm-db fleet_tailscale repo (store the
  user's Tailscale API key + tailnet, server-side only).
- cm-api routes/tailscale.rs: POST/GET/DELETE /api/fleet/tailscale + GET
  /api/fleet/tailscale/devices (proxies api.tailscale.com device list).
- Daemon: --tailscale-authkey → `tailscale up --authkey … --ssh` (enables
  Tailscale SSH for keyless user access); else `tailscale set --ssh=true`. Reports
  its tailscale IP (already).

UI:
- Fleet overview gains a Tailscale section: connect (key+tailnet) + live tailnet
  device status (online/last-seen/IP/os). Node cards show a copyable Tailscale SSH
  target (ssh <ip>).

Remaining: P2 — RemoteDriver + placement (run agents on nodes) and the in-UI
remote terminal (PTY proxied over the WSS channel).

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

crates/cm-api/src/routes/nodes.rs

@@ -109,12 +109,7 @@ pub async fn exec_test(
     let node = nodes::get(&state.pool, node_id, user.workspace_id)
         .await?
         .ok_or(ApiError::NotFound)?;
-    let cmd = vec![
-        "sh".to_owned(),
-        "-lc".to_owned(),
-        "uname -a; echo '---'; docker version --format 'docker {{.Server.Version}}' 2>/dev/null || echo 'docker: not found'".to_owned(),
-    ];
-    match state.node_hub.exec(node_id, &cmd).await {
+    match state.node_hub.verify(node_id).await {
         Ok(out) => Ok(Json(json!({ "ok": out.ok, "output": out.output, "node": node.name }))),
         Err(e) => Ok(Json(json!({ "ok": false, "output": e, "node": node.name }))),
     }