refactor(auth): remove obsolete CONVERT_PASSWORDS env var

Legacy md5/argon2id hashes are now always upgraded to bcrypt on login, so
the CONVERT_PASSWORDS flag is no longer used. Drop it from env schema,
.env.example, the docker installer, and test mocks.

src/lib/auth/password.test.ts

@@ -138,14 +138,14 @@ describe("isDoubleMd5Of", () => {
 describe("isSaltedMd5Of", () => {
 	it("verifies md5(salt+password) with hash:salt layout", async () => {
 		const salt = "pepper123";
-		const stored = `${(await md5Hex(salt + "oldpass"))}:${salt}`;
+		const stored = `${await md5Hex(salt + "oldpass")}:${salt}`;
 		expect(await isSaltedMd5Of("oldpass", stored)).toBe(true);
 		expect(await isSaltedMd5Of("wrong", stored)).toBe(false);
 	});
 
 	it("verifies md5(password+salt) with hash:salt layout", async () => {
 		const salt = "pepper123";
-		const stored = `${(await md5Hex("oldpass" + salt))}:${salt}`;
+		const stored = `${await md5Hex("oldpass" + salt)}:${salt}`;
 		expect(await isSaltedMd5Of("oldpass", stored)).toBe(true);
 	});