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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.

Merge functional changes from develop branch:
- Remove AntigravityQuotaScope system (claude/gemini_text/gemini_image)
- Replace with per-model rate limiting using resolveAntigravityModelKey
- Remove model load statistics (IncrModelCallCount/GetModelLoadBatch)
- Simplify account selection to unified priority→load→LRU algorithm
- Remove SetAntigravityQuotaScopeLimit from AccountRepository
- Clean up scope-related UI indicators and API fields

Key changes:
- Upgrade model mapping: Opus 4.5 → Opus 4.6-thinking with precise matching
- Unified rate limiting: scope-level → model-level with Redis snapshot sync
- Load-balanced scheduling by call count with smart retry mechanism
- Force cache billing support
- Model identity injection in prompts with leak prevention
- Thinking mode auto-handling (max_tokens/budget_tokens fix)
- Frontend: whitelist mode toggle, model mapping validation, status indicators
- Gemini session fallback with Redis Trie O(L) matching
- Ops: enhanced concurrency monitoring, account availability, retry logic
- Migration scripts: 049-051 for model mapping unification

在运维监控的并发/排队卡片中，为 Antigravity 平台账号显示各 scope
(claude/gemini_text/gemini_image) 的限流数量统计，便于管理员了解
哪些 scope 正在被限流。

Cherry-picked from slovx2/sub2api: 08d6dc52

在运维监控的并发/排队卡片中，为 Antigravity 平台账号显示各 scope
(claude/gemini_text/gemini_image) 的限流数量统计，便于管理员了解
哪些 scope 正在被限流。

* fix: 添加 gemini-3-flash 前缀映射支持 gemini-3-flash-preview

* feat(antigravity): 增强请求参数和注入 Antigravity 身份 system prompt

* feat: antigravity 配额域限流

* chore: 调整 SSE 单行上限到 25MB

* chore: 提升 SSE 单行上限到 40MB