fix(channelserver): return error from Save() to prevent misleading success logs

CharacterSaveData.Save() silently returned on failure (nil decompressed
data, compression error, DB error) while the caller unconditionally
logged "Saved character data successfully". This made diagnosing save
failures difficult (ref #163).

Save() now returns an error, and all six call sites check it. The
success log in saveAllCharacterData only fires when the save actually
persisted.

server/channelserver/handlers_character_test.go

@@ -476,7 +476,9 @@ func TestCharacterSaveData_Save_Integration(t *testing.T) {
 	saveData.WeaponID = 1234
 
 	// Save it
-	saveData.Save(s)
+	if err := saveData.Save(s); err != nil {
+		t.Fatalf("Save failed: %v", err)
+	}
 
 	// Reload and verify
 	var hr, gr uint16