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/.env.production.example

@@ -50,3 +50,11 @@ LOCKOUT_DURATION_MINUTES=15
 
 # Logging
 LOG_LEVEL=warn
+
+# Scheduler
+# Enable simple in-process scheduler for auto-matching (enable on exactly one replica)
+ENABLE_SCHEDULER=false
+# Global tick interval in seconds (e.g., 300 = 5min)
+SCHEDULER_INTERVAL_SEC=300
+# Per-event minimum time between runs in seconds to avoid thrashing
+MATCHING_MIN_INTERVAL_SEC=120