feat(backend): presence stats count time online from sessions in the browser time zone, capture uptime marks hours without data, presence reaches every stream of the account, message volume keeps empty days

backend/src/api/timezone.py

@@ -0,0 +1,10 @@
+import asyncpg
+from fastapi import HTTPException
+
+from utils.read.period import known_timezone
+
+
+async def checked_tz(pool: asyncpg.Pool, tz: str) -> str:
+    if not await known_timezone(pool, tz):
+        raise HTTPException(status_code=422, detail=f"Unknown time zone: {tz}")
+    return tz