refactor: replace sync.Map credits state with AICredits rate limit key

Replace process-memory sync.Map + per-model runtime state with a single
"AICredits" key in model_rate_limits, making credits exhaustion fully
isomorphic with model-level rate limiting.

Scheduler: rate-limited accounts with overages enabled + credits available
are now scheduled instead of excluded.

Forwarding: when model is rate-limited + credits available, inject credits
proactively without waiting for a 429 round trip.

Storage: credits exhaustion stored as model_rate_limits["AICredits"] with
5h duration, reusing SetModelRateLimit/isRateLimitActiveForKey.

Frontend: show credits_active (yellow ⚡) when model rate-limited but
credits available, credits_exhausted (red) when AICredits key active.

Tests: add unit tests for shouldMarkCreditsExhausted, injectEnabledCreditTypes,
clearCreditsExhausted, and update existing overages tests.

frontend/src/components/account/__tests__/AccountStatusIndicator.spec.ts

@@ -43,17 +43,18 @@ function makeAccount(overrides: Partial<Account>): Account {
 }
 
 describe('AccountStatusIndicator', () => {
-  it('会将超量请求中的模型显示为独立状态', () => {
+  it('模型限流 + overages 启用 + 无 AICredits key → 显示 ⚡ (credits_active)', () => {
     const wrapper = mount(AccountStatusIndicator, {
       props: {
         account: makeAccount({
           id: 1,
           name: 'ag-1',
           extra: {
-            antigravity_credits_overages: {
+            allow_overages: true,
+            model_rate_limits: {
               'claude-sonnet-4-5': {
-                activated_at: '2026-03-15T00:00:00Z',
-                active_until: '2099-03-15T00:00:00Z'
+                rate_limited_at: '2026-03-15T00:00:00Z',
+                rate_limit_reset_at: '2099-03-15T00:00:00Z'
               }
             }
           }
@@ -70,7 +71,7 @@ describe('AccountStatusIndicator', () => {
     expect(wrapper.text()).toContain('CSon45')
   })
 
-  it('普通模型限流仍显示原有限流状态', () => {
+  it('模型限流 + overages 未启用 → 普通限流样式（无 ⚡）', () => {
     const wrapper = mount(AccountStatusIndicator, {
       props: {
         account: makeAccount({
@@ -96,4 +97,66 @@ describe('AccountStatusIndicator', () => {
     expect(wrapper.text()).toContain('CSon45')
     expect(wrapper.text()).not.toContain('⚡')
   })
+
+  it('AICredits key 生效 → 显示积分已用尽 (credits_exhausted)', () => {
+    const wrapper = mount(AccountStatusIndicator, {
+      props: {
+        account: makeAccount({
+          id: 3,
+          name: 'ag-3',
+          extra: {
+            allow_overages: true,
+            model_rate_limits: {
+              'AICredits': {
+                rate_limited_at: '2026-03-15T00:00:00Z',
+                rate_limit_reset_at: '2099-03-15T00:00:00Z'
+              }
+            }
+          }
+        })
+      },
+      global: {
+        stubs: {
+          Icon: true
+        }
+      }
+    })
+
+    expect(wrapper.text()).toContain('account.creditsExhausted')
+  })
+
+  it('模型限流 + overages 启用 + AICredits key 生效 → 普通限流样式（积分耗尽，无 ⚡）', () => {
+    const wrapper = mount(AccountStatusIndicator, {
+      props: {
+        account: makeAccount({
+          id: 4,
+          name: 'ag-4',
+          extra: {
+            allow_overages: true,
+            model_rate_limits: {
+              'claude-sonnet-4-5': {
+                rate_limited_at: '2026-03-15T00:00:00Z',
+                rate_limit_reset_at: '2099-03-15T00:00:00Z'
+              },
+              'AICredits': {
+                rate_limited_at: '2026-03-15T00:00:00Z',
+                rate_limit_reset_at: '2099-03-15T00:00:00Z'
+              }
+            }
+          }
+        })
+      },
+      global: {
+        stubs: {
+          Icon: true
+        }
+      }
+    })
+
+    // 模型限流 + 积分耗尽 → 不应显示 ⚡
+    expect(wrapper.text()).toContain('CSon45')
+    expect(wrapper.text()).not.toContain('⚡')
+    // AICredits 积分耗尽状态应显示
+    expect(wrapper.text()).toContain('account.creditsExhausted')
+  })
 })