feat: add app tier system — core/recommended/optional (SCALE-02, SCALE-03)

get_app_tier() classifies all apps:
- core: Bitcoin, LND, Electrs, Mempool, BTCPay, DWN, FileBrowser
- recommended: Fedimint, Grafana, Vaultwarden, Kuma, SearXNG, etc.
- optional: everything else

Tier field added to Manifest struct (data_model.rs) and exposed
via WebSocket package data for frontend tier badges.

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

core/archipelago/src/data_model.rs

@@ -150,6 +150,9 @@ pub struct Manifest {
     pub author: Option<String>,
     pub website: Option<String>,
     pub interfaces: Option<Interfaces>,
+    /// App tier: "core", "recommended", or "optional"
+    #[serde(default)]
+    pub tier: Option<String>,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]