fix(billing): hold inflight reservation until billing lands; price estimate like billing; wire remaining endpoints

Address gaps found in adversarial verification of the in-flight balance
reservation:

- Early release: the reservation is now a ref-counted handle carried on the
  request context. Billing tasks submitted via submit*UsageRecordTask take a
  ref and return it only after RecordUsage has run; with reservation enabled
  the balance cache is deducted synchronously inside the billing task, so the
  reservation is released only after admission can see the deduction. Dropped
  tasks return their ref immediately. The reservation is renewed every ttl/3
  while the handler runs; after the handler returns it is no longer renewed,
  so the post-handler hold is bounded by one TTL.
- Estimate: use the same billing model (channel mapping + BillingModelSource,
  max over requested/mapped when ambiguous) and ModelPricingResolver (group
  ModelPricing incl. wildcards, channel per_request/image/video/token
  overrides), default/group/user rate + peak multiplier, plus image, video,
  audio and search surcharge kinds. Unpriced requests fail open with a
  throttled log; new billing.inflight_reservation.fail_closed_on_unpriced
  (default false) rejects them instead.
- Wire images, embeddings, web/x search, alpha search, grok media generation,
  grok realtime/voice, OpenAI-platform /v1/messages and ResponsesWebSocket
  (session-level reservation with renewal).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

backend/internal/handler/gemini_v1beta_handler.go

@@ -391,7 +391,7 @@ func (h *GatewayHandler) GeminiV1BetaModels(c *gin.Context) {
 	}
 
 	// 余额模式在途预留：防止并发请求在预检时看到同一份余额而集体透支。
-	inflightRelease, err := reserveInflightBalance(c.Request.Context(), h.billingCacheService, h.gatewayService, apiKey, subscription, modelName, body)
+	inflightRelease, err := reserveInflightBalance(c, h.billingCacheService, h.gatewayService, apiKey, subscription, tokenInflightEstimate(modelName, body))
 	if err != nil {
 		reqLog.Info("gemini.inflight_reservation_rejected", zap.Error(err))
 		status, _, message, retryAfter := billingErrorDetails(err)