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/prisma/schema.prisma

@@ -165,17 +165,24 @@ model Match {
   user1LastReadAt DateTime? @map("user1_last_read_at")
   user2LastReadAt DateTime? @map("user2_last_read_at")
 
+  // Auto-matching integration
+  suggestionId    Int?      @unique @map("suggestion_id") // Link to auto-matching suggestion (optional)
+  source          String    @default("manual") @db.VarChar(20) // 'manual' | 'auto'
+  statsApplied    Boolean   @default(false) @map("stats_applied") // Flag to ensure stats applied only once
+
   // Relations
-  user1     User      @relation("MatchUser1", fields: [user1Id], references: [id])
-  user2     User      @relation("MatchUser2", fields: [user2Id], references: [id])
-  event     Event     @relation(fields: [eventId], references: [id])
-  room      ChatRoom? @relation(fields: [roomId], references: [id])
-  ratings   Rating[]
+  user1      User                 @relation("MatchUser1", fields: [user1Id], references: [id])
+  user2      User                 @relation("MatchUser2", fields: [user2Id], references: [id])
+  event      Event                @relation(fields: [eventId], references: [id])
+  room       ChatRoom?            @relation(fields: [roomId], references: [id])
+  suggestion RecordingSuggestion? @relation(fields: [suggestionId], references: [id])
+  ratings    Rating[]
 
   @@unique([user1Id, user2Id, eventId])
   @@index([user1Id])
   @@index([user2Id])
   @@index([eventId])
+  @@index([suggestionId])
   @@map("matches")
 }
 
@@ -285,6 +292,7 @@ model RecordingSuggestion {
   event    Event          @relation(fields: [eventId], references: [id], onDelete: Cascade)
   heat     EventUserHeat  @relation(fields: [heatId], references: [id], onDelete: Cascade)
   recorder User?          @relation("RecorderAssignments", fields: [recorderId], references: [id])
+  match    Match?         // Link to created match (if suggestion was accepted)
 
   @@index([eventId])
   @@index([recorderId])