fix(auth): race-safe first-run setup + rolling cookie max-age refresh

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/data/users.test.ts

@@ -72,4 +72,24 @@ describe("UserStore", () => {
     expect(users.countUsers()).toBe(0);
     expect(users.deleteUser("not-a-real-id")).toBe(false);
   });
+
+  it("createFirstUser succeeds on empty db, returns null when a user already exists", async () => {
+    const a = await users.createFirstUser("alice", "pw-alice");
+    expect(a).not.toBeNull();
+    expect(a!.username).toBe("alice");
+    const b = await users.createFirstUser("bob", "pw-bob-bob");
+    expect(b).toBeNull();
+    expect(users.countUsers()).toBe(1);
+  });
+
+  it("createFirstUser is race-safe: concurrent calls produce exactly one user", async () => {
+    const [a, b, c] = await Promise.all([
+      users.createFirstUser("alice", "pw-alice"),
+      users.createFirstUser("bob", "pw-bob-bob"),
+      users.createFirstUser("charlie", "pw-charlie-pw"),
+    ]);
+    const created = [a, b, c].filter((u) => u !== null);
+    expect(created).toHaveLength(1);
+    expect(users.countUsers()).toBe(1);
+  });
 });

src/data/users.ts

@@ -15,6 +15,7 @@ export interface UserRow {
 export interface UserStore {
   countUsers(): number;
   createUser(username: string, password: string): Promise<UserRow>;
+  createFirstUser(username: string, password: string): Promise<UserRow | null>;
   findByUsername(username: string): UserRow | null;
   findById(id: string): UserRow | null;
   verifyPassword(plain: string, hash: string): Promise<boolean>;
@@ -69,6 +70,26 @@ export function createUserStore(db: Database.Database): UserStore {
       return { id, username, passwordHash: hash, createdAt: now, updatedAt: now };
     },
 
+    async createFirstUser(username, password) {
+      const hash = await bcrypt.hash(password, BCRYPT_ROUNDS);
+      const id = randomUUID();
+      const now = Date.now();
+      const run = db.transaction(() => {
+        const count = (countStmt.get() as { n: number }).n;
+        if (count !== 0) return null;
+        try {
+          insertStmt.run(id, username, hash, now, now);
+        } catch (err) {
+          if (err && typeof err === "object" && (err as { code?: string }).code === "SQLITE_CONSTRAINT_UNIQUE") {
+            return null;
+          }
+          throw err;
+        }
+        return { id, username, passwordHash: hash, createdAt: now, updatedAt: now } as UserRow;
+      });
+      return run();
+    },
+
     findByUsername(username) {
       return (findByUsernameStmt.get(username) as UserRow | undefined) ?? null;
     },