feat: expose lastMonthRating and deltaPredicted on player objects (#3)

Add two derived fields to all player objects returned by
getPlayerDataFromDB and the error branch in getAllRatingsFromDB.
No new DB columns — both fields are pure arithmetic derivations.
monthlyHistory placeholder [] included ahead of A2 implementation.

src/services/player-service.js

@@ -18,13 +18,23 @@ async function getPlayerDataFromDB(pdgaNumber) {
         stdDev = updatedPlayer?.std_dev;
       }
 
+      const rating = cachedPlayer.current_rating;
+      const ratingChange = cachedPlayer.rating_change;
+      const resolvedPredicted = predictedRating > 0 ? predictedRating : null;
+      const resolvedStdDev = stdDev > 0 ? stdDev : null;
+
       return {
         pdgaNumber: cachedPlayer.pdga_number,
         name: cachedPlayer.name,
-        rating: cachedPlayer.current_rating,
-        ratingChange: cachedPlayer.rating_change,
-        predictedRating: predictedRating > 0 ? predictedRating : null,
-        stdDev: stdDev > 0 ? stdDev : null
+        rating,
+        ratingChange,
+        predictedRating: resolvedPredicted,
+        stdDev: resolvedStdDev,
+        // previous month's official rating (null when either value is missing)
+        lastMonthRating: (rating != null && ratingChange != null) ? rating - ratingChange : null,
+        // gap between next predicted update and current rating (null when either is missing)
+        deltaPredicted: (resolvedPredicted != null && rating != null) ? resolvedPredicted - rating : null,
+        monthlyHistory: []
       };
     }
     return null;
@@ -161,12 +171,18 @@ async function getAllRatingsFromDB(progressCallback = null) {
         }
       } catch (error) {
         logger.error(`Failed to load PDGA ${pdgaNumber} from database:`, error.message);
+        const errorRating = player.current_rating;
+        const errorRatingChange = player.rating_change;
         const errorData = {
           pdgaNumber: parseInt(pdgaNumber),
           name: player.name || 'Database Error',
-          rating: player.current_rating,
-          ratingChange: player.rating_change,
-          predictedRating: null
+          rating: errorRating,
+          ratingChange: errorRatingChange,
+          predictedRating: null,
+          stdDev: null,
+          lastMonthRating: (errorRating != null && errorRatingChange != null) ? errorRating - errorRatingChange : null,
+          deltaPredicted: null,
+          monthlyHistory: []
         };
         ratings.push(errorData);