perf(cache): tell a stale record from its header instead of parsing it (#633)

The sports plugins cache whole season schedules: 53MB for MLB, 18MB for
NHL, 17MB for NCAA baseball. On a Pi 4, orjson.loads of the MLB file
takes ~1.8s with the GIL held, and every thread in the display service
waits -- the stall watchdog caught the render thread frozen 0.5-1.3s with
the interpreter itself blocked, right on these reads. When a season record
expired, DiskCache.get paid that whole parse only to find the timestamp
too old and throw the result away.

CacheManager.set now writes timestamp and ttl ahead of the data, and
DiskCache.get reads them from the first 256 bytes of the file, applying
the same rule as before (a per-entry ttl wins over max_age; no limit
means never stale). A record that is stale is refused without being
parsed. Files in the old layout, and records from other writers, don't
match the header and are parsed in full as before.

Also: ESPN responses in the background data service and espn_dates are
parsed with orjson when it is installed (src/common/json_body.py). The
stdlib parser behind response.json() takes 3.1s on the MLB season
against orjson's 1.8s, both with the GIL held. espn_dates imports it with
a fallback, since plugins bundle copies of that module for older cores.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

src/common/espn_dates.py

@@ -39,6 +39,14 @@ from datetime import date, timedelta
 from functools import partial
 from typing import Any, Dict, List, Optional, Tuple
 
+try:
+    from src.common.json_body import response_json
+except ImportError:
+    # Plugins bundle copies of this module for older cores, which predate
+    # json_body; the stdlib parse is what those cores always used.
+    def response_json(response: Any) -> Any:
+        return response.json()
+
 # Above this, ESPN returns a truncated list instead of an error. See module
 # docstring: 500 is the largest value measured to return complete data.
 ESPN_MAX_LIMIT = 500
@@ -194,7 +202,7 @@ def _fetch_one_chunk(
             timeout=timeout,
         )
         response.raise_for_status()
-        return response.json()
+        return response_json(response)
     except Exception as exc:  # noqa: BLE001 - see docstring
         if logger:
             logger.warning("ESPN chunk %s failed, skipping it: %s", chunk, exc)
@@ -371,4 +379,4 @@ def fetch_espn_scoreboard(
         if data is not None:
             return data
     response.raise_for_status()
-    return response.json()
+    return response_json(response)