feat(usage): add request concurrency trends (#537)

* feat(usage): add request concurrency trends

* perf(usage)!: unify load insights and optimize concurrency

Aggregate concurrency on a single event timeline and reuse lifecycle
indexes instead of joining every request against every time bucket.

Align load summaries and trend points with the four chart metrics,
combine concurrency and scheduling in one view, and right-align the
statistics card selectors. Update the API contract and regression tests.

BREAKING CHANGE: Usage insights concurrency and scheduling P95 fields
move under performance.load and performance.points[].load. Unused
scheduling P50, capacity, and performance coverage fields are removed.

Co-authored-by: Codex <noreply@openai.com>

---------

Co-authored-by: Zyy <zyycn@users.noreply.github.com>
Co-authored-by: Codex <noreply@openai.com>

frontend/src/api/modules/usage.ts

@@ -256,47 +256,32 @@ export interface UsageOverviewHealth {
   points: UsageOverviewHealthPoint[]
 }
 
-export interface UsageOverviewPerformancePoint {
-  bucket: string
-  label: string
-  latencyP50Ms: number | null
-  latencyP95Ms: number | null
-  latencyP99Ms: number | null
-  firstTokenP50Ms: number | null
-  firstTokenP95Ms: number | null
-  firstTokenP99Ms: number | null
-  admissionDecisionP50Ms: number | null
+export interface UsageOverviewLoad {
+  averageConcurrency: number
+  peakConcurrency: number
   admissionDecisionP95Ms: number | null
-  accountSelectionWaitP50Ms: number | null
   accountSelectionWaitP95Ms: number | null
-  outputThroughputP10: number | null
-  outputThroughputP50: number | null
-  outputThroughputP90: number | null
-  capacityUtilization: number | null
-  capacityUtilizationP95: number | null
 }
 
-export interface UsageOverviewPerformance {
+export interface UsageOverviewPerformanceMetrics {
   latencyP50Ms: number | null
   latencyP95Ms: number | null
   latencyP99Ms: number | null
   firstTokenP50Ms: number | null
   firstTokenP95Ms: number | null
   firstTokenP99Ms: number | null
-  admissionDecisionP50Ms: number | null
-  admissionDecisionP95Ms: number | null
-  accountSelectionWaitP50Ms: number | null
-  accountSelectionWaitP95Ms: number | null
   outputThroughputP10: number | null
   outputThroughputP50: number | null
   outputThroughputP90: number | null
-  capacityUtilization: number | null
-  capacityUtilizationP95: number | null
-  latencyCoverage: number
-  firstTokenCoverage: number
-  admissionDecisionCoverage: number
-  accountSelectionWaitCoverage: number
-  capacityCoverage: number
+  load: UsageOverviewLoad
+}
+
+export interface UsageOverviewPerformancePoint extends UsageOverviewPerformanceMetrics {
+  bucket: string
+  label: string
+}
+
+export interface UsageOverviewPerformance extends UsageOverviewPerformanceMetrics {
   points: UsageOverviewPerformancePoint[]
 }