feat(parser): add ISO8601 duration parser

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/unit/iso8601-duration.test.ts

@@ -0,0 +1,23 @@
+import { describe, it, expect } from 'vitest';
+import { parseIso8601Duration } from '../../src/lib/server/parsers/iso8601-duration';
+
+describe('parseIso8601Duration', () => {
+  it.each([
+    ['PT30M', 30],
+    ['PT1H', 60],
+    ['PT1H30M', 90],
+    ['PT2H15M', 135],
+    ['PT0M', 0],
+    ['P1DT2H', 26 * 60],
+    ['PT90M', 90]
+  ] as const)('parses %s to %i minutes', (input, expected) => {
+    expect(parseIso8601Duration(input)).toBe(expected);
+  });
+
+  it.each([[''], [null], [undefined], ['garbage'], ['30 min'], ['PT']])(
+    'returns null for invalid input %j',
+    (input) => {
+      expect(parseIso8601Duration(input as string)).toBeNull();
+    }
+  );
+});