Use crypto.randomBytes for session tokens; add deleteOtherSessions helper

Session tokens were derived from Math.random() + Date.now() — predictable
enough that a determined attacker could brute-force or guess valid
tokens for other users. Switch to crypto.randomBytes(32) (256 bits of
CSPRNG output, hex-encoded), the standard for opaque bearer tokens.

Also adds deleteOtherSessions(userId, keepToken) so the upcoming
password-change flow can log a user out of every other device when
they rotate their password.

proof-of-work/lib/auth.ts

@@ -1,4 +1,5 @@
 import bcryptjs from "bcryptjs";
+import { randomBytes } from "node:crypto";
 import { prisma } from "./prisma";
 import { ReadonlyRequestCookies } from "next/dist/server/web/spec-extension/adapters/request-cookies";
 import { cookies } from "next/headers";
@@ -23,14 +24,16 @@ export async function verifyPassword(
 }
 
 /**
- * Create a session token for a user (30-day expiration)
+ * Create a session token for a user (30-day expiration).
+ *
+ * Token is 256 bits of CSPRNG output, hex-encoded (64 chars). Do not
+ * weaken this — predictable tokens enable cross-user impersonation,
+ * and under multi-user that means anyone-can-be-anyone if guessable.
  */
 export async function createSession(
   userId: string
 ): Promise<{ token: string; expiresAt: Date }> {
-  const token = Buffer.from(
-    `${userId}:${Date.now()}:${Math.random()}`
-  ).toString("hex");
+  const token = randomBytes(32).toString("hex");
 
   const expiresAt = new Date(Date.now() + 30 * 24 * 60 * 60 * 1000); // 30 days
 
@@ -76,6 +79,25 @@ export async function deleteSession(token: string): Promise<void> {
   });
 }
 
+/**
+ * Revoke every session for a user except the optional `keepToken`.
+ * Used by password-change to log the user out of every other device
+ * when they rotate their password (defense against compromised
+ * sessions).
+ */
+export async function deleteOtherSessions(
+  userId: string,
+  keepToken: string | null,
+): Promise<number> {
+  const result = await prisma.session.deleteMany({
+    where: {
+      userId,
+      ...(keepToken ? { NOT: { token: keepToken } } : {}),
+    },
+  });
+  return result.count;
+}
+
 /**
  * Get session from cookies object
  */