fix: house_system param encoding for swe.houses()

src/ephemeris_mcp/server.py

@@ -468,7 +468,7 @@ def get_sky_state(
 
     # Compute house cusps when requested
     if house_system is not None:
-        hs_code = house_system.upper().encode()
+        hs_code = house_system.upper().encode()[:1]  # single byte: b'P', b'K', etc.
         try:
             cusps, ascmc = swe.houses(jd, lat, lon, hs_code)
             houses = []