feat(antigravity): comprehensive enhancements - model mapping, rate limiting, scheduling & ops

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

backend/migrations/051_migrate_opus45_to_opus46_thinking.sql

@@ -0,0 +1,41 @@
+-- Migrate all Opus 4.5 models to Opus 4.6-thinking
+--
+-- Background:
+-- Antigravity now supports claude-opus-4-6-thinking and no longer supports opus-4-5
+--
+-- Strategy:
+-- Directly overwrite the entire model_mapping with updated mappings
+-- This ensures consistency with DefaultAntigravityModelMapping in constants.go
+
+UPDATE accounts
+SET credentials = jsonb_set(
+    credentials,
+    '{model_mapping}',
+    '{
+        "claude-opus-4-6-thinking": "claude-opus-4-6-thinking",
+        "claude-opus-4-6": "claude-opus-4-6-thinking",
+        "claude-opus-4-5-thinking": "claude-opus-4-6-thinking",
+        "claude-opus-4-5-20251101": "claude-opus-4-6-thinking",
+        "claude-sonnet-4-5": "claude-sonnet-4-5",
+        "claude-sonnet-4-5-thinking": "claude-sonnet-4-5-thinking",
+        "claude-sonnet-4-5-20250929": "claude-sonnet-4-5",
+        "claude-haiku-4-5": "claude-sonnet-4-5",
+        "claude-haiku-4-5-20251001": "claude-sonnet-4-5",
+        "gemini-2.5-flash": "gemini-2.5-flash",
+        "gemini-2.5-flash-lite": "gemini-2.5-flash-lite",
+        "gemini-2.5-flash-thinking": "gemini-2.5-flash-thinking",
+        "gemini-2.5-pro": "gemini-2.5-pro",
+        "gemini-3-flash": "gemini-3-flash",
+        "gemini-3-pro-high": "gemini-3-pro-high",
+        "gemini-3-pro-low": "gemini-3-pro-low",
+        "gemini-3-pro-image": "gemini-3-pro-image",
+        "gemini-3-flash-preview": "gemini-3-flash",
+        "gemini-3-pro-preview": "gemini-3-pro-high",
+        "gemini-3-pro-image-preview": "gemini-3-pro-image",
+        "gpt-oss-120b-medium": "gpt-oss-120b-medium",
+        "tab_flash_lite_preview": "tab_flash_lite_preview"
+    }'::jsonb
+)
+WHERE platform = 'antigravity'
+  AND deleted_at IS NULL
+  AND credentials->'model_mapping' IS NOT NULL;