feat(scheduler): in-process matching scheduler with audit + admin endpoints

- Add in-process scheduler service triggered by ENABLE_SCHEDULER
- Record runs in new matching_runs table; throttle per-event and log stats
- Add admin endpoints: POST /api/admin/events/:slug/run-now and GET /api/admin/events/:slug/matching-runs
- Wire scheduler start/stop in server and add ENV flags + compose defaults
- Prisma schema: add MatchingRun model and relation
- Update env examples for scheduler configuration

backend/prisma/schema.prisma

@@ -102,6 +102,7 @@ model Event {
   checkinToken         EventCheckinToken?
   userHeats            EventUserHeat[]
   recordingSuggestions RecordingSuggestion[]
+  matchingRuns         MatchingRun[]
 
   @@map("events")
 }
@@ -298,3 +299,22 @@ model RecordingSuggestion {
   @@index([recorderId])
   @@map("recording_suggestions")
 }
+
+// Matching runs audit log
+model MatchingRun {
+  id             Int      @id @default(autoincrement())
+  eventId        Int      @map("event_id")
+  trigger        String   @db.VarChar(20) // 'manual' | 'scheduler'
+  status         String   @default("running") @db.VarChar(20) // 'running' | 'success' | 'error'
+  startedAt      DateTime @default(now()) @map("started_at")
+  endedAt        DateTime? @map("ended_at")
+  matchedCount   Int      @default(0) @map("matched_count")
+  notFoundCount  Int      @default(0) @map("not_found_count")
+  error          String?  @db.Text
+
+  // Relations
+  event          Event    @relation(fields: [eventId], references: [id], onDelete: Cascade)
+
+  @@index([eventId, startedAt])
+  @@map("matching_runs")
+}