perf: cache public activity aggregates

apps/api/app/public_cache.py

@@ -0,0 +1,32 @@
+from __future__ import annotations
+
+from copy import deepcopy
+from threading import Lock
+from time import monotonic
+from typing import Any
+
+
+class PublicResponseCache:
+    def __init__(self) -> None:
+        self._items: dict[tuple[Any, ...], tuple[float, Any]] = {}
+        self._lock = Lock()
+
+    def get(self, key: tuple[Any, ...], ttl_seconds: int) -> Any | None:
+        with self._lock:
+            item = self._items.get(key)
+            if item is None or monotonic() - item[0] >= ttl_seconds:
+                self._items.pop(key, None)
+                return None
+            return deepcopy(item[1])
+
+    def set(self, key: tuple[Any, ...], value: Any) -> Any:
+        with self._lock:
+            self._items[key] = (monotonic(), deepcopy(value))
+        return value
+
+    def invalidate(self) -> None:
+        with self._lock:
+            self._items.clear()
+
+
+public_cache = PublicResponseCache()