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/050_map_opus46_to_opus45.sql

@@ -0,0 +1,17 @@
+-- Map claude-opus-4-6 to claude-opus-4-5-thinking
+--
+-- Notes:
+-- - Updates existing Antigravity accounts' model_mapping
+-- - Changes claude-opus-4-6 target from claude-opus-4-6 to claude-opus-4-5-thinking
+-- - This is needed because previous versions didn't have this mapping
+
+UPDATE accounts
+SET credentials = jsonb_set(
+    credentials,
+    '{model_mapping,claude-opus-4-6}',
+    '"claude-opus-4-5-thinking"'::jsonb
+)
+WHERE platform = 'antigravity'
+  AND deleted_at IS NULL
+  AND credentials->'model_mapping' IS NOT NULL
+  AND credentials->'model_mapping'->>'claude-opus-4-6' IS NOT NULL;