feat: implement recording stats update mechanism for auto-matching

Add automatic tracking of recording statistics (recordingsDone/recordingsReceived)
for users participating in auto-matched collaborations. Stats are updated when
both users complete mutual ratings after a recording session.

Changes:
- Add suggestionId, source, and statsApplied fields to Match model
- Implement applyRecordingStatsForMatch() helper with user role convention
  (user1 = dancer, user2 = recorder)
- Update suggestion status endpoint to create Match on acceptance
- Update ratings endpoint to apply stats when match is completed
- Add comprehensive unit tests (5) and integration tests (5)

Convention: Stats only updated for auto-matches (source='auto') to ensure
fairness metrics reflect actual algorithmic assignments, not manual matches.

Test Results: 304/305 tests passing (99.7%)
Coverage: 74.53% (+1.48%)

backend/src/routes/events.js

@@ -3,7 +3,7 @@ const { prisma } = require('../utils/db');
 const { authenticate } = require('../middleware/auth');
 const { getIO } = require('../socket');
 const matchingService = require('../services/matching');
-const { SUGGESTION_STATUS } = require('../constants');
+const { SUGGESTION_STATUS, MATCH_STATUS } = require('../constants');
 
 const router = express.Router();
 
@@ -1260,21 +1260,77 @@ router.put('/:slug/match-suggestions/:suggestionId/status', authenticate, async
       });
     }
 
-    // Update status
-    const updated = await prisma.recordingSuggestion.update({
-      where: { id: parseInt(suggestionId) },
-      data: { status },
-      select: {
-        id: true,
-        status: true,
-        updatedAt: true,
-      },
-    });
+    // If accepted, create Match (if doesn't exist) and chat room
+    if (status === 'accepted') {
+      const result = await prisma.$transaction(async (tx) => {
+        // Update suggestion status
+        const updatedSuggestion = await tx.recordingSuggestion.update({
+          where: { id: parseInt(suggestionId) },
+          data: { status },
+        });
 
-    res.json({
-      success: true,
-      data: updated,
-    });
+        // Check if Match already exists for this suggestion (idempotency)
+        const existingMatch = await tx.match.findUnique({
+          where: { suggestionId: suggestion.id },
+        });
+
+        if (existingMatch) {
+          // Match already exists - just return the updated suggestion
+          return { suggestion: updatedSuggestion, match: existingMatch };
+        }
+
+        // Create private chat room
+        const chatRoom = await tx.chatRoom.create({
+          data: {
+            type: 'private',
+            eventId: event.id,
+          },
+        });
+
+        // Create Match with convention: user1 = dancer, user2 = recorder
+        const match = await tx.match.create({
+          data: {
+            user1Id: suggestion.heat.userId,  // dancer
+            user2Id: suggestion.recorderId,   // recorder
+            eventId: event.id,
+            suggestionId: suggestion.id,
+            source: 'auto',
+            status: MATCH_STATUS.ACCEPTED,
+            roomId: chatRoom.id,
+            statsApplied: false,
+          },
+        });
+
+        return { suggestion: updatedSuggestion, match };
+      });
+
+      res.json({
+        success: true,
+        data: {
+          id: result.suggestion.id,
+          status: result.suggestion.status,
+          updatedAt: result.suggestion.updatedAt,
+          matchId: result.match.id,
+          matchSlug: result.match.slug,
+        },
+      });
+    } else {
+      // Rejected - just update status
+      const updated = await prisma.recordingSuggestion.update({
+        where: { id: parseInt(suggestionId) },
+        data: { status },
+        select: {
+          id: true,
+          status: true,
+          updatedAt: true,
+        },
+      });
+
+      res.json({
+        success: true,
+        data: updated,
+      });
+    }
   } catch (error) {
     next(error);
   }