Add Rust server

Owns the database, PIN rotation and all LiveKit token minting. Kiosk
registration, session-stable joins, admin auth, layouts, branding, media
uploads and the hosted installer script.

Claude-Session: https://claude.ai/code/session_01SS9F92jb51bMCRCKem6QtD

server/migrations/0003_settings.sql

@@ -0,0 +1,5 @@
+CREATE TABLE IF NOT EXISTS settings (
+    key TEXT PRIMARY KEY,
+    value TEXT NOT NULL,
+    updated_at INTEGER NOT NULL
+);