feat(backend): persistent settings store + settingsFile config

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/config.ts

@@ -3,6 +3,7 @@ export interface AppConfig {
   authMode: "none" | "mock" | "oidc";
   defaults: { provider?: string; model?: string; thinking?: string };
   maxPiProcesses: number;
+  settingsFile: string;
 }
 export function loadConfig(env: Record<string, string | undefined>): AppConfig {
   return {
@@ -13,5 +14,6 @@ export function loadConfig(env: Record<string, string | undefined>): AppConfig {
     authMode: (env.AUTH_MODE as AppConfig["authMode"]) ?? "none",
     defaults: { provider: env.PI_PROVIDER, model: env.PI_MODEL, thinking: env.PI_THINKING },
     maxPiProcesses: Number(env.MAX_PI_PROCESSES ?? 4),
+    settingsFile: env.SETTINGS_FILE ?? "data/settings.json",
   };
 }