feat(auth): switch password hashing to argon2id with legacy auto-upgrade

- hashPassword now emits argon2id (same params as the legacy AtomCMS
  Laravel setup: memory 64MB, iterations 4, parallelism 1)
- legacy md5 and bcrypt hashes are verified and auto-upgraded to
  argon2id on successful login (CONVERT_PASSWORDS=true)
- replace BCRYPT_ROUNDS env with ARGON2_MEMORY_KB / ARGON2_ITERATIONS /
  ARGON2_PARALLELISM
- update README and add tests for argon2id and bcrypt upgrade paths

src/lib/auth/password.test.ts

@@ -1,7 +1,9 @@
 import { describe, expect, it, vi } from "vitest";
 
 const mockEnv = vi.hoisted(() => ({
-	BCRYPT_ROUNDS: 12,
+	ARGON2_MEMORY_KB: 65_536,
+	ARGON2_ITERATIONS: 4,
+	ARGON2_PARALLELISM: 1,
 }));
 
 vi.mock("@/env", () => ({
@@ -11,6 +13,8 @@ vi.mock("@/env", () => ({
 import {
 	checkLogin,
 	hashPassword,
+	isArgon2idOf,
+	isBcryptOf,
 	isMd5Of,
 	md5Hex,
 	verifyPassword,
@@ -24,20 +28,37 @@ describe("md5Hex", () => {
 });
 
 describe("hashPassword", () => {
-	it("emits a bcrypt hash and round-trips", async () => {
+	it("emits an argon2id hash and round-trips", async () => {
 		const h = await hashPassword("s3cret!");
-		expect(h).toMatch(/^\$2y\$\d{2}\$/);
+		expect(h).toMatch(/^\$argon2id\$/);
 		expect(await verifyPassword("s3cret!", h)).toBe(true);
 		expect(await verifyPassword("wrong", h)).toBe(false);
 	});
 });
 
-describe("verifyPassword", () => {
-	it("verifies legacy bcrypt hashes ($2y$)", async () => {
-		const h = await hashPassword("hunter2");
-		expect(h).toMatch(/^\$2y\$/);
-		expect(await verifyPassword("hunter2", h)).toBe(true);
-		expect(await verifyPassword("nope", h)).toBe(false);
+describe("isArgon2idOf", () => {
+	it("verifies an argon2id hash (AtomCMS/Laravel)", async () => {
+		const stored =
+			"$argon2id$v=19$m=1024,t=1,p=1$pTCeoGfX788sH7Z3ju9rJw$4awmR4yciu2L+xDNQJ/NesWX3Kio+fwN8wSCtp4XUp0";
+		expect(await isArgon2idOf("test-password-123", stored)).toBe(true);
+		expect(await isArgon2idOf("wrong", stored)).toBe(false);
+		expect(await isArgon2idOf("anything", "$2y$12$ABC")).toBe(false);
+	});
+});
+
+describe("isBcryptOf", () => {
+	it("verifies a legacy bcrypt hash", async () => {
+		const { bcrypt } = await import("hash-wasm");
+		const { randomBytes } = await import("node:crypto");
+		const stored = await bcrypt({
+			password: "hunter2",
+			salt: randomBytes(16),
+			costFactor: 10,
+			outputType: "encoded",
+		});
+		expect(await isBcryptOf("hunter2", stored)).toBe(true);
+		expect(await isBcryptOf("wrong", stored)).toBe(false);
+		expect(await isBcryptOf("anything", "$argon2id$v=19$")).toBe(false);
 	});
 });
 
@@ -49,12 +70,21 @@ describe("isMd5Of", () => {
 	});
 });
 
+describe("verifyPassword", () => {
+	it("verifies argon2id hashes", async () => {
+		const h = await hashPassword("hunter2");
+		expect(h).toMatch(/^\$argon2id\$/);
+		expect(await verifyPassword("hunter2", h)).toBe(true);
+		expect(await verifyPassword("nope", h)).toBe(false);
+	});
+});
+
 describe("checkLogin", () => {
-	it("upgrades a legacy md5 hash to bcrypt when conversion is enabled", async () => {
+	it("upgrades a legacy md5 hash to argon2id when conversion is enabled", async () => {
 		const stored = await md5Hex("oldpass");
 		const res = await checkLogin("oldpass", stored, { convertPasswords: true });
 		expect(res.valid).toBe(true);
-		expect(res.upgradedHash).toMatch(/^\$2y\$/);
+		expect(res.upgradedHash).toMatch(/^\$argon2id\$/);
 		expect(await verifyPassword("oldpass", res.upgradedHash as string)).toBe(
 			true,
 		);
@@ -69,6 +99,35 @@ describe("checkLogin", () => {
 		expect(res.upgradedHash).toBeUndefined();
 	});
 
+	it("accepts an argon2id hash with no rehash", async () => {
+		const stored =
+			"$argon2id$v=19$m=1024,t=1,p=1$pTCeoGfX788sH7Z3ju9rJw$4awmR4yciu2L+xDNQJ/NesWX3Kio+fwN8wSCtp4XUp0";
+		const res = await checkLogin("test-password-123", stored, {
+			convertPasswords: true,
+		});
+		expect(res.valid).toBe(true);
+		expect(res.upgradedHash).toBeUndefined();
+	});
+
+	it("upgrades a legacy bcrypt hash to argon2id when conversion is enabled", async () => {
+		// Generate a real bcrypt hash via hash-wasm and verify the upgrade path.
+		const { bcrypt } = await import("hash-wasm");
+		const stored = await bcrypt({
+			password: "oldbcrypt",
+			salt: await import("node:crypto").then((c) => c.randomBytes(16)),
+			costFactor: 10,
+			outputType: "encoded",
+		});
+		const res = await checkLogin("oldbcrypt", stored, {
+			convertPasswords: true,
+		});
+		expect(res.valid).toBe(true);
+		expect(res.upgradedHash).toMatch(/^\$argon2id\$/);
+		expect(await verifyPassword("oldbcrypt", res.upgradedHash as string)).toBe(
+			true,
+		);
+	});
+
 	it("validates an existing modern hash with no upgrade", async () => {
 		const stored = await hashPassword("modern");
 		const res = await checkLogin("modern", stored, { convertPasswords: true });