Intro skip: MAL timings via keyless Jikan + player button

- upstream: _intros(mal_id) -> per-episode {start,length} (seconds),
  30d TTL cache; Jikan is down or show uncovered -> [] (no button)
- app: GET /api/intros/{mal_id}, never errors into the player
- player: 'skip intro' pill appears while currentTime is inside the
  intro window (+8s grace); click seeks past; reappears if rewound.
  Ep-1 intros are often missing in MAL data, so falls back to the
  show's first known timing (usually identical across eps)
- test: _parse_intros unit test (Jikan itself is 504ing right now,
  live path verified by hand: endpoint returns [] cleanly)

tests/test_api.py

@@ -296,3 +296,19 @@ def _search_count(user):
     n = c.execute("SELECT COUNT(*) FROM searches WHERE user=?", (user,)).fetchone()[0]
     c.close()
     return n
+
+
+def test_parse_intros():
+    from server.upstream import _parse_intros
+    out = _parse_intros([
+        {"intro": {"start": 95, "length": 60}},
+        {"intro": {"start": 0, "length": 0}},   # MAL "no intro" marker
+        {"intro": None},
+        {},
+    ])
+    assert out == [
+        {"start": 95, "length": 60},
+        {"start": 0, "length": 0},
+        None,
+        None,
+    ]