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.ts

@@ -57,7 +57,10 @@ export async function isDoubleMd5Of(
 	stored: string,
 ): Promise<boolean> {
 	if (!/^[a-f0-9]{32}$/i.test(stored)) return false;
-	return (await md5Hex(await md5Hex(password))).toLowerCase() === stored.toLowerCase();
+	return (
+		(await md5Hex(await md5Hex(password))).toLowerCase() ===
+		stored.toLowerCase()
+	);
 }
 
 /**