perf(cache): skip rewriting unchanged CacheManager.set() records (#730)

The disk cache's unchanged-payload skip now ignores a CacheManager.set() record's timestamp, so unchanged re-saves are skipped; a skip moves the file's mtime to the new timestamp instead, and readers take a record's age from the newer of the two (never more than an hour past the embedded timestamp). Per-plugin plugin_metrics:<id> records become one plugin_metrics_snapshot written at most once a minute, and CacheManager builds its ConfigManager on first use. On hdpi, cache file writes went from ~37 to 8.6 a minute.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/test_cache_manager_lazy_config.py

@@ -0,0 +1,98 @@
+"""CacheManager builds its ConfigManager on first use, not in __init__.
+
+Every CacheManager built a ConfigManager and loaded the whole config for a
+cache strategy that no longer reads it. The attribute stays public -- the
+sports plugins resolve the global timezone through
+``cache_manager.config_manager`` -- so it is now built on first access.
+"""
+
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+import src.config_manager as config_manager_module
+from src.cache_manager import CacheManager
+
+
+@pytest.fixture
+def built(monkeypatch):
+    """Count ConfigManager constructions and load_config calls."""
+    made = []
+
+    class CountingConfigManager:
+        def __init__(self):
+            made.append(self)
+            self.loads = 0
+
+        def load_config(self):
+            self.loads += 1
+            return {}
+
+    monkeypatch.setattr(config_manager_module, "ConfigManager", CountingConfigManager)
+    return made
+
+
+@pytest.fixture
+def manager(tmp_path):
+    with patch('src.cache_manager.CacheManager._get_writable_cache_dir',
+               return_value=str(tmp_path)):
+        cm = CacheManager()
+    cm.stop_cleanup_thread()
+    return cm
+
+
+def test_construction_does_not_load_the_config(built, manager):
+    manager.set("k", {"v": 1})
+    assert manager.get("k") == {"v": 1}
+    assert manager.get_cache_strategy("sports_live")["max_age"] > 0
+    assert built == []
+
+
+def test_first_access_builds_and_loads_it_once(built, manager):
+    first = manager.config_manager
+    assert manager.config_manager is first
+    assert getattr(manager, "config_manager", None) is first
+    assert len(built) == 1 and first.loads == 1
+
+
+def test_assignment_still_wins(built, manager):
+    replacement = MagicMock()
+    manager.config_manager = replacement
+    assert manager.config_manager is replacement
+    assert built == []
+
+
+def test_an_unimportable_config_manager_is_none(manager, monkeypatch):
+    import builtins
+    real_import = builtins.__import__
+
+    def refuse(name, *args, **kwargs):
+        if name == "src.config_manager":
+            raise ImportError("no config manager here")
+        return real_import(name, *args, **kwargs)
+
+    monkeypatch.setattr(builtins, "__import__", refuse)
+    assert manager.config_manager is None
+
+
+def test_a_failed_load_is_retried_on_the_next_access(manager, monkeypatch):
+    attempts = []
+
+    class Flaky:
+        def load_config(self):
+            attempts.append(1)
+            if len(attempts) == 1:
+                raise RuntimeError("config.json unreadable")
+            return {}
+
+    monkeypatch.setattr(config_manager_module, "ConfigManager", Flaky)
+    with pytest.raises(RuntimeError):
+        manager.config_manager
+    assert isinstance(manager.config_manager, Flaky)
+    assert len(attempts) == 2
+
+
+def test_a_manager_made_without_init_still_answers(built):
+    bare = CacheManager.__new__(CacheManager)
+    bare.logger = MagicMock()
+    assert bare.config_manager is built[0]