fix: resolve 5 audit findings in channel/credits/scheduling

P0-1: Credits degraded response retry + fail-open
- Add isAntigravityDegradedResponse() to detect transient API failures
- Retry up to 3 times with exponential backoff (500ms/1s/2s)
- Invalidate singleflight cache between retries
- Fail-open after exhausting retries instead of 5h circuit break

P1-1: Fix channel restriction pre-check timing conflict
- Swap checkClaudeCodeRestriction before checkChannelPricingRestriction
- Ensures channel restriction is checked against final fallback groupID

P1-2: Add interval pricing validation (frontend + backend)
- Backend: ValidateIntervals() with boundary, price, overlap checks
- Frontend: validateIntervals() with Chinese error messages
- Rules: MinTokens>=0, MaxTokens>MinTokens, prices>=0, no overlap

P2: Fix cross-platform same-model pricing/mapping override
- Store cache keys using original platform instead of group platform
- Lookup across matching platforms (antigravity→anthropic→gemini)
- Prevents anthropic/gemini same-name models from overwriting each other

frontend/src/components/admin/channel/types.ts

@@ -113,6 +113,70 @@ export function findModelConflict(models: string[]): [string, string] | null {
   return null
 }
 
+// ── 区间校验 ──────────────────────────────────────────────
+
+/** 校验区间列表的合法性，返回错误消息；通过则返回 null */
+export function validateIntervals(intervals: IntervalFormEntry[]): string | null {
+  if (!intervals || intervals.length === 0) return null
+
+  // 按 min_tokens 排序（不修改原数组）
+  const sorted = [...intervals].sort((a, b) => a.min_tokens - b.min_tokens)
+
+  for (let i = 0; i < sorted.length; i++) {
+    const err = validateSingleInterval(sorted[i], i)
+    if (err) return err
+  }
+  return checkIntervalOverlap(sorted)
+}
+
+function validateSingleInterval(iv: IntervalFormEntry, idx: number): string | null {
+  if (iv.min_tokens < 0) {
+    return `区间 #${idx + 1}: 最小 token 数 (${iv.min_tokens}) 不能为负数`
+  }
+  if (iv.max_tokens != null) {
+    if (iv.max_tokens <= 0) {
+      return `区间 #${idx + 1}: 最大 token 数 (${iv.max_tokens}) 必须大于 0`
+    }
+    if (iv.max_tokens <= iv.min_tokens) {
+      return `区间 #${idx + 1}: 最大 token 数 (${iv.max_tokens}) 必须大于最小 token 数 (${iv.min_tokens})`
+    }
+  }
+  return validateIntervalPrices(iv, idx)
+}
+
+function validateIntervalPrices(iv: IntervalFormEntry, idx: number): string | null {
+  const prices: [string, number | string | null][] = [
+    ['输入价格', iv.input_price],
+    ['输出价格', iv.output_price],
+    ['缓存写入价格', iv.cache_write_price],
+    ['缓存读取价格', iv.cache_read_price],
+    ['单次价格', iv.per_request_price],
+  ]
+  for (const [name, val] of prices) {
+    if (val != null && val !== '' && Number(val) < 0) {
+      return `区间 #${idx + 1}: ${name}不能为负数`
+    }
+  }
+  return null
+}
+
+function checkIntervalOverlap(sorted: IntervalFormEntry[]): string | null {
+  for (let i = 0; i < sorted.length; i++) {
+    // 无上限区间必须是最后一个
+    if (sorted[i].max_tokens == null && i < sorted.length - 1) {
+      return `区间 #${i + 1}: 无上限区间（最大 token 数为空）只能是最后一个`
+    }
+    if (i === 0) continue
+    const prev = sorted[i - 1]
+    // (min, max] 语义：前一个区间上界 > 当前区间下界则重叠
+    if (prev.max_tokens == null || prev.max_tokens > sorted[i].min_tokens) {
+      const prevMax = prev.max_tokens == null ? '∞' : String(prev.max_tokens)
+      return `区间 #${i} 和 #${i + 1} 重叠：前一个区间上界 (${prevMax}) 大于当前区间下界 (${sorted[i].min_tokens})`
+    }
+  }
+  return null
+}
+
 /** 平台对应的模型 tag 样式（背景+文字） */
 export function getPlatformTagClass(platform: string): string {
   switch (platform) {