feat(auth): add UserStore with bcryptjs password hashing

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

src/data/users.test.ts

@@ -0,0 +1,56 @@
+import { describe, it, expect, beforeEach, afterEach } from "vitest";
+import { createDatabase, type BotDatabase } from "./database.js";
+import { createUserStore, UsernameTakenError, type UserStore } from "./users.js";
+
+describe("UserStore", () => {
+  let botDb: BotDatabase;
+  let users: UserStore;
+
+  beforeEach(() => {
+    botDb = createDatabase(":memory:");
+    users = createUserStore(botDb.db);
+  });
+
+  afterEach(() => {
+    botDb.close();
+  });
+
+  it("countUsers is 0 on a fresh db", () => {
+    expect(users.countUsers()).toBe(0);
+  });
+
+  it("createUser stores the user and bumps countUsers", async () => {
+    const u = await users.createUser("alice", "pw-hunter2");
+    expect(u.id).toMatch(/^[0-9a-f-]{36}$/);
+    expect(u.username).toBe("alice");
+    expect(users.countUsers()).toBe(1);
+  });
+
+  it("findByUsername is case-insensitive and returns null for missing", async () => {
+    await users.createUser("Alice", "pw");
+    expect(users.findByUsername("ALICE")).not.toBeNull();
+    expect(users.findByUsername("alice")).not.toBeNull();
+    expect(users.findByUsername("bob")).toBeNull();
+  });
+
+  it("createUser rejects duplicate usernames (case-insensitive)", async () => {
+    await users.createUser("Alice", "pw");
+    await expect(users.createUser("alice", "pw2")).rejects.toBeInstanceOf(UsernameTakenError);
+  });
+
+  it("verifyPassword accepts correct password and rejects wrong one", async () => {
+    await users.createUser("alice", "correct-horse-battery-staple");
+    const row = users.findByUsername("alice");
+    expect(row).not.toBeNull();
+    expect(await users.verifyPassword("correct-horse-battery-staple", row!.passwordHash)).toBe(true);
+    expect(await users.verifyPassword("wrong", row!.passwordHash)).toBe(false);
+  });
+
+  it("changePassword updates the hash so the old password no longer verifies", async () => {
+    const u = await users.createUser("alice", "old");
+    await users.changePassword(u.id, "new");
+    const row = users.findByUsername("alice");
+    expect(await users.verifyPassword("old", row!.passwordHash)).toBe(false);
+    expect(await users.verifyPassword("new", row!.passwordHash)).toBe(true);
+  });
+});