refactor: switch password hashing from argon2 to bcrypt

- hashPassword now emits bcrypt (cost 12) instead of argon2id
- checkLogin migrates legacy md5/argon2id hashes to bcrypt on sign-in
- keep argon2id verification only as a one-time migration path
- replace ARGON2_* env vars with BCRYPT_COST

src/lib/auth/password.test.ts

@@ -1,9 +1,7 @@
 import { describe, expect, it, vi } from "vitest";
 
 const mockEnv = vi.hoisted(() => ({
-	ARGON2_MEMORY_KB: 65_536,
-	ARGON2_ITERATIONS: 4,
-	ARGON2_PARALLELISM: 1,
+	BCRYPT_COST: 12,
 }));
 
 vi.mock("@/env", () => ({
@@ -28,16 +26,16 @@ describe("md5Hex", () => {
 });
 
 describe("hashPassword", () => {
-	it("emits an argon2id hash and round-trips", async () => {
+	it("emits a bcrypt hash and round-trips", async () => {
 		const h = await hashPassword("s3cret!");
-		expect(h).toMatch(/^\$argon2id\$/);
+		expect(h).toMatch(/^\$2[aby]\$/);
 		expect(await verifyPassword("s3cret!", h)).toBe(true);
 		expect(await verifyPassword("wrong", h)).toBe(false);
 	});
 });
 
 describe("isArgon2idOf", () => {
-	it("verifies an argon2id hash (AtomCMS/Laravel)", async () => {
+	it("verifies a legacy argon2id hash (pre-migration accounts)", 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);
@@ -47,7 +45,7 @@ describe("isArgon2idOf", () => {
 });
 
 describe("isBcryptOf", () => {
-	it("verifies a legacy bcrypt hash", async () => {
+	it("verifies a bcrypt hash", async () => {
 		const { bcrypt } = await import("hash-wasm");
 		const { randomBytes } = await import("node:crypto");
 		const stored = await bcrypt({
@@ -71,20 +69,20 @@ describe("isMd5Of", () => {
 });
 
 describe("verifyPassword", () => {
-	it("verifies argon2id hashes", async () => {
+	it("verifies bcrypt hashes", async () => {
 		const h = await hashPassword("hunter2");
-		expect(h).toMatch(/^\$argon2id\$/);
+		expect(h).toMatch(/^\$2[aby]\$/);
 		expect(await verifyPassword("hunter2", h)).toBe(true);
 		expect(await verifyPassword("nope", h)).toBe(false);
 	});
 });
 
 describe("checkLogin", () => {
-	it("upgrades a legacy md5 hash to argon2id when conversion is enabled", async () => {
+	it("upgrades a legacy md5 hash to bcrypt 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(/^\$argon2id\$/);
+		expect(res.upgradedHash).toMatch(/^\$2[aby]\$/);
 		expect(await verifyPassword("oldpass", res.upgradedHash as string)).toBe(
 			true,
 		);
@@ -99,37 +97,25 @@ describe("checkLogin", () => {
 		expect(res.upgradedHash).toBeUndefined();
 	});
 
-	it("accepts an argon2id hash with no rehash", async () => {
+	it("migrates a legacy argon2id hash to bcrypt", 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();
+		expect(res.upgradedHash).toMatch(/^\$2[aby]\$/);
 	});
 
-	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.
+	it("accepts an existing bcrypt hash with no rehash", async () => {
 		const { bcrypt } = await import("hash-wasm");
+		const { randomBytes } = await import("node:crypto");
 		const stored = await bcrypt({
-			password: "oldbcrypt",
-			salt: await import("node:crypto").then((c) => c.randomBytes(16)),
+			password: "modern",
+			salt: 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 });
 		expect(res.valid).toBe(true);
 		expect(res.upgradedHash).toBeUndefined();