fix(scheduler): resolve SetSnapshot race conditions and remove usage throttle

Backend: Fix three race conditions in SetSnapshot that caused account
scheduling anomalies and broken sticky sessions:
- Use Lua CAS script for atomic version activation, preventing version
  rollback when concurrent goroutines write snapshots simultaneously
- Add UnlockBucket to release rebuild lock immediately after completion
  instead of waiting 30s TTL expiry
- Replace immediate DEL of old snapshots with 60s EXPIRE grace period,
  preventing readers from hitting empty ZRANGE during version switches

Frontend: Remove serial queue throttle (1-2s delay per request) from
usage loading since backend now uses passive sampling. All usage
requests execute immediately in parallel.

frontend/src/utils/usageLoadQueue.ts

@@ -1,93 +1,18 @@
 /**
- * Usage request scheduler — throttles Anthropic API calls by proxy exit.
+ * Usage request scheduler.
  *
- * Anthropic OAuth/setup-token accounts sharing the same proxy exit are placed
- * into a serial queue with a random 1–2s delay between requests, preventing
- * upstream 429 rate-limit errors.
- *
- * Proxy identity = host:port:username — two proxy records pointing to the
- * same exit share a single queue. Accounts without a proxy go into a
- * "direct" queue.
- *
- * All other platforms bypass the queue and execute immediately.
+ * All platforms execute immediately without queuing — the backend uses
+ * passive sampling so upstream 429 rate-limit errors are no longer a concern.
  */
 
 import type { Account } from '@/types'
 
-const GROUP_DELAY_MIN_MS = 1000
-const GROUP_DELAY_MAX_MS = 2000
-
-type Task<T> = {
-  fn: () => Promise<T>
-  resolve: (value: T) => void
-  reject: (reason: unknown) => void
-}
-
-const queues = new Map<string, Task<unknown>[]>()
-const running = new Set<string>()
-
-/** Whether this account needs throttled queuing. */
-function needsThrottle(account: Account): boolean {
-  return (
-    account.platform === 'anthropic' &&
-    (account.type === 'oauth' || account.type === 'setup-token')
-  )
-}
-
-/** Build a queue key from proxy connection details. */
-function buildGroupKey(account: Account): string {
-  const proxy = account.proxy
-  const proxyIdentity = proxy
-    ? `${proxy.host}:${proxy.port}:${proxy.username || ''}`
-    : 'direct'
-  return `anthropic:${proxyIdentity}`
-}
-
-async function drain(groupKey: string) {
-  if (running.has(groupKey)) return
-  running.add(groupKey)
-
-  const queue = queues.get(groupKey)
-  while (queue && queue.length > 0) {
-    const task = queue.shift()!
-    try {
-      const result = await task.fn()
-      task.resolve(result)
-    } catch (err) {
-      task.reject(err)
-    }
-    if (queue.length > 0) {
-      const jitter = GROUP_DELAY_MIN_MS + Math.random() * (GROUP_DELAY_MAX_MS - GROUP_DELAY_MIN_MS)
-      await new Promise((r) => setTimeout(r, jitter))
-    }
-  }
-
-  running.delete(groupKey)
-  queues.delete(groupKey)
-}
-
 /**
- * Schedule a usage fetch. Anthropic accounts are queued by proxy exit;
- * all other platforms execute immediately.
+ * Schedule a usage fetch. All requests execute immediately.
  */
 export function enqueueUsageRequest<T>(
-  account: Account,
+  _account: Account,
   fn: () => Promise<T>
 ): Promise<T> {
-  // Non-Anthropic → fire immediately, no queuing
-  if (!needsThrottle(account)) {
-    return fn()
-  }
-
-  const key = buildGroupKey(account)
-
-  return new Promise<T>((resolve, reject) => {
-    let queue = queues.get(key)
-    if (!queue) {
-      queue = []
-      queues.set(key, queue)
-    }
-    queue.push({ fn, resolve, reject } as Task<unknown>)
-    drain(key)
-  })
+  return fn()
 }