fix: remove hardcoded JWT secret fallback from createBetterAuthInstance

.env.example

@@ -1,3 +1,4 @@
 DATABASE_URL=postgres://paperclip:paperclip@localhost:5432/paperclip
 PORT=3100
 SERVE_UI=false
+BETTER_AUTH_SECRET=paperclip-dev-secret

server/src/auth/better-auth.ts

@@ -67,7 +67,13 @@ export function deriveAuthTrustedOrigins(config: Config): string[] {
 
 export function createBetterAuthInstance(db: Db, config: Config, trustedOrigins?: string[]): BetterAuthInstance {
   const baseUrl = config.authBaseUrlMode === "explicit" ? config.authPublicBaseUrl : undefined;
-  const secret = process.env.BETTER_AUTH_SECRET ?? process.env.PAPERCLIP_AGENT_JWT_SECRET ?? "paperclip-dev-secret";
+  const secret = process.env.BETTER_AUTH_SECRET ?? process.env.PAPERCLIP_AGENT_JWT_SECRET;
+  if (!secret) {
+    throw new Error(
+      "BETTER_AUTH_SECRET (or PAPERCLIP_AGENT_JWT_SECRET) must be set. " +
+      "For local development, set BETTER_AUTH_SECRET=paperclip-dev-secret in your .env file.",
+    );
+  }
   const effectiveTrustedOrigins = trustedOrigins ?? deriveAuthTrustedOrigins(config);
 
   const publicUrl = process.env.PAPERCLIP_PUBLIC_URL ?? baseUrl;

server/src/index.ts

@@ -475,13 +475,6 @@ export async function startServer(): Promise<StartedServer> {
       resolveBetterAuthSession,
       resolveBetterAuthSessionFromHeaders,
     } = await import("./auth/better-auth.js");
-    const betterAuthSecret =
-      process.env.BETTER_AUTH_SECRET?.trim() ?? process.env.PAPERCLIP_AGENT_JWT_SECRET?.trim();
-    if (!betterAuthSecret) {
-      throw new Error(
-        "authenticated mode requires BETTER_AUTH_SECRET (or PAPERCLIP_AGENT_JWT_SECRET) to be set",
-      );
-    }
     const derivedTrustedOrigins = deriveAuthTrustedOrigins(config);
     const envTrustedOrigins = (process.env.BETTER_AUTH_TRUSTED_ORIGINS ?? "")
       .split(",")