fix(ops): increase error log request body limit from 10KB to 256KB

10KB is too aggressive for modern LLM API requests where conversation
context routinely exceeds 1MB. This causes error logs to contain only
a minimal placeholder, making it impossible to debug upstream failures.

256KB retains enough context for effective debugging while the existing
multi-pass trimming logic handles larger payloads gracefully.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/internal/service/ops_service.go

@@ -16,7 +16,7 @@ import (
 var ErrOpsDisabled = infraerrors.NotFound("OPS_DISABLED", "Ops monitoring is disabled")
 
 const (
-	opsMaxStoredRequestBodyBytes = 10 * 1024
+	opsMaxStoredRequestBodyBytes = 256 * 1024
 	opsMaxStoredErrorBodyBytes   = 20 * 1024
 )