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/views/admin/ChannelsView.vue

@@ -418,7 +418,7 @@ import { useAppStore } from '@/stores/app'
 import { adminAPI } from '@/api/admin'
 import type { Channel, ChannelModelPricing, CreateChannelRequest, UpdateChannelRequest } from '@/api/admin/channels'
 import type { PricingFormEntry } from '@/components/admin/channel/types'
-import { mTokToPerToken, perTokenToMTok, apiIntervalsToForm, formIntervalsToAPI, findModelConflict } from '@/components/admin/channel/types'
+import { mTokToPerToken, perTokenToMTok, apiIntervalsToForm, formIntervalsToAPI, findModelConflict, validateIntervals } from '@/components/admin/channel/types'
 import type { AdminGroup, GroupPlatform } from '@/types'
 import type { Column } from '@/components/common/types'
 import AppLayout from '@/components/layout/AppLayout.vue'
@@ -922,6 +922,21 @@ async function handleSubmit() {
     }
   }
 
+  // 校验区间合法性（范围、重叠等）
+  for (const section of form.platforms.filter(s => s.enabled)) {
+    for (const entry of section.model_pricing) {
+      if (!entry.intervals || entry.intervals.length === 0) continue
+      const intervalErr = validateIntervals(entry.intervals)
+      if (intervalErr) {
+        const platformLabel = t('admin.groups.platforms.' + section.platform, section.platform)
+        const modelLabel = entry.models.join(', ') || '未命名'
+        appStore.showError(`${platformLabel} - ${modelLabel}: ${intervalErr}`)
+        activeTab.value = section.platform
+        return
+      }
+    }
+  }
+
   const { group_ids, model_pricing, model_mapping } = formToAPI()
 
   submitting.value = true