perf(sports): fetch ESPN date chunks concurrently

Since ESPN started rejecting `dates=YYYYMMDD-YYYYMMDD` on 2026-09-15, one
season request became a chunk per month -- and a month over the 500-event
cap becomes a request per day. A cold college-baseball season is about 130
requests, and they went out one at a time.

That is slower than the 20s budget `_update_plugins()` shares across every
plugin at startup, so scoreboards were logging `update() timed out` on
first run and being deferred to the scheduled tick with nothing on the
panel. Measured on a Pi 4 against live ESPN, March+April college baseball
(63 requests, 3101 events): 11.2s sequential, 1.6s concurrent. Over a whole
boot that moved football-scoreboard, ledmatrix-flights and birdnet-go
inside the budget -- 13 plugins deferred before, 10 after.

Chunks now go out six at a time, in two passes: months and edge days first,
then the days of any month that came back capped. Six keeps the shared
Session under requests' default pool_maxsize of 10, so no connection is
discarded. Merged events still follow `espn_date_chunks` order -- a capped
month's days are spliced back into its own slot -- so the payload does not
depend on which request won the race.

Request order is no longer significant, so the three tests that pinned it
compare the chunks as a set and keep asserting the merged event order,
which is the part callers actually see.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

test/test_background_data_service_espn_ranges.py

@@ -115,8 +115,11 @@ def test_a_rejected_season_is_recovered_and_cached(service, cache):
 def test_a_full_season_costs_chunks_not_one_request_per_day(service):
     session = RangeRejectingSession({"202609": [{"id": "a"}]})
     submit_and_wait(service, session, "20260801-20270301")
-    assert [call["dates"] for call in session.calls] == [
-        "20260801-20270301",
+    sent = [call["dates"] for call in session.calls]
+    # Eight chunks rather than 213 per-day requests. They are fetched
+    # concurrently, so the range is the only one pinned to a position.
+    assert sent[0] == "20260801-20270301"
+    assert sorted(sent[1:]) == [
         "202608",
         "202609",
         "202610",