fix(sse): scale chat admission by ingest byte budget instead of a fixed request count (#11548)

Merged via /merge-batch (lote 2026-08-26, v3.8.51). Boarded no worktree combinado junto com outras ~30 PRs; validação única: typecheck/complexity/cognitive-complexity/changelog-integrity verdes, file-size rebaseado onde necessário (crescimento legítimo), lint com os mesmos 228 achados pré-existentes confirmados via sonda contra o tip puro (não introduzidos por este lote), e ~370 testes focados (unit + vitest) passando. Obrigado pela contribuição.

.env.example

@@ -403,8 +403,17 @@ ALLOW_API_KEY_REVEAL=false
 # OMNIROUTE_CHAT_LARGE_BODY_BYTES=262144
 # Actual-byte hard cap enforced during bounded ingestion. Default 52428800 (50 MB).
 # OMNIROUTE_CHAT_HARD_MAX_BODY_BYTES=52428800
-# Maximum heavyweight requests simultaneously admitted in one process. Default 1.
+# Legacy request-COUNT cap (#503-fanout). Now binds only when explicitly set here —
+# left unset, heavyweight admission is gated by OMNIROUTE_CHAT_MAX_INFLIGHT_BYTES below
+# instead (an auto-derived byte budget), fixing coding-agent fan-out (multiple
+# subagents/CLIs) collapsing to an effective concurrency of ~1 and 503ing.
 # OMNIROUTE_CHAT_MAX_HEAVY_IN_FLIGHT=1
+# Override for the auto-derived ingest byte budget (#503-fanout). Default: 25% of the
+# process's effective memory ceiling (V8 heap limit, or the tighter cgroup/container
+# limit) divided by an 8x transient-amplification factor, clamped between 8 MiB and
+# 2 GiB; explicit overrides are clamped to the same safe range. Read
+# chatAdmission.maxInflightBytes/budgetSource at /api/monitoring/health before overriding.
+# OMNIROUTE_CHAT_MAX_INFLIGHT_BYTES=134217728
 # Heap-pressure shed ratio (heapUsed/heap_size_limit) for the structural admission gate
 # (#10183, #10268): a second concurrent heavyweight request past OMNIROUTE_CHAT_MAX_HEAVY_IN_FLIGHT
 # is only shed with a retryable 503 when the heap is ALSO under this much pressure — on a