feat(matching-runs): attach origin_run_id to new suggestions and expose pairs-per-run API

- Extend saveMatchingResults(eventId, suggestions, runId) and set originRunId
- Scheduler/Admin run-now: always pass runId
- Admin API: GET /api/admin/events/:slug/matching-runs/:runId/suggestions
- Prisma: add compound index on (origin_run_id, status)
- Frontend: add getRunSuggestions, expand row in MatchingRunsSection with 'Pairs created in this run' wording

backend/prisma/schema.prisma

@@ -286,6 +286,7 @@ model RecordingSuggestion {
   heatId     Int      @unique @map("heat_id") // One suggestion per heat
   recorderId Int?     @map("recorder_id")     // NULL if no match found
   status     String   @default("pending") @db.VarChar(20) // 'pending', 'accepted', 'rejected', 'not_found'
+  originRunId Int?    @map("origin_run_id")
   createdAt  DateTime @default(now()) @map("created_at")
   updatedAt  DateTime @updatedAt @map("updated_at")
 
@@ -294,9 +295,11 @@ model RecordingSuggestion {
   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)
+  originRun MatchingRun?  @relation(fields: [originRunId], references: [id])
 
   @@index([eventId])
   @@index([recorderId])
+  @@index([originRunId])
   @@map("recording_suggestions")
 }