fix: align web API limit cap with 10k and clean up postgres_history imports

src/web_api.py

@@ -452,7 +452,7 @@ async def get_thaiwater_sensors():
 async def get_postgres_history(
     station_code: str,
     hours: int = Query(168, ge=1),
-    limit: int = Query(2000, ge=1, le=5000),
+    limit: int = Query(2000, ge=1, le=10000),
 ):
     """Get historical measurements for a station from PostgreSQL."""
     cache_key = f"{station_code}:{hours}:{limit}"