feat(web): cache dashboard aggregations with version invalidation

reaction_board/repository.py

@@ -555,6 +555,43 @@ class Repository:
             (key, str(value)),
         ))
 
+    # ------------------------------------------------------------ cache version
+
+    @staticmethod
+    def cache_version_key(group: str) -> str:
+        return f"cache:v:{group}"
+
+    def cache_version(self, group: str) -> int:
+        def run(conn: sqlite3.Connection) -> int:
+            row = conn.execute(
+                "SELECT v FROM kv WHERE k = ?", (self.cache_version_key(group),)
+            ).fetchone()
+            if row is None:
+                return 0
+            try:
+                return int(str(row["v"]))
+            except ValueError:
+                return 0
+        return self.db.read(run)
+
+    def bump_cache_version(self, group: str, *, now: int | None = None) -> int:
+        now = self._now() if now is None else int(now)
+        key = self.cache_version_key(group)
+
+        def run(conn: sqlite3.Connection) -> int:
+            row = conn.execute("SELECT v FROM kv WHERE k = ?", (key,)).fetchone()
+            try:
+                value = int(str(row["v"])) + 1 if row is not None else 1
+            except ValueError:
+                value = 1
+            conn.execute(
+                "INSERT INTO kv(k, v) VALUES (?, ?) "
+                "ON CONFLICT(k) DO UPDATE SET v = excluded.v",
+                (key, str(value)),
+            )
+            return value
+        return self.db.write(run)
+
     # ------------------------------------------------------------ candidate
 
     def known_aid(self, aid: int) -> bool: