feat(webapp): enable combined concurrency limit enforcement by default

The env var stays as a kill switch: set
RUN_ENGINE_TOTAL_CONCURRENCY_LIMITS_ENABLED=0 to disable enforcement
without redeploying code. Off by default previously meant the SDK,
dashboard, and API surfaced combined limits that nothing enforced.

Author: matt-aitken

apps/webapp/app/env.server.ts

@@ -1397,7 +1397,7 @@ const EnvironmentSchema = z
     RUN_ENGINE_RUN_QUEUE_LOG_LEVEL: z
       .enum(["log", "error", "warn", "info", "debug"])
       .default("info"),
-    RUN_ENGINE_TOTAL_CONCURRENCY_LIMITS_ENABLED: z.string().default("0"),
+    RUN_ENGINE_TOTAL_CONCURRENCY_LIMITS_ENABLED: z.string().default("1"),
     RUN_ENGINE_TREAT_PRODUCTION_EXECUTION_STALLS_AS_OOM: z.string().default("0"),
     RUN_ENGINE_READ_REPLICA_SNAPSHOTS_SINCE_ENABLED: z.string().default("0"),
     RUN_ENGINE_SNAPSHOTS_SINCE_REPLICA_RETRY_MIN_MS: z.coerce.number().int().default(50),