feat(strategy): add quota-awareness to hybrid account selection

The hybrid strategy now considers account quota levels when selecting
accounts, preventing any single account from being drained to 0%.

- Add QuotaTracker class to track per-account quota levels
- Exclude accounts with critical quota (<5%) from selection
- Add quota component to scoring formula (weight: 3)
- Fall back to critical accounts when no alternatives exist
- Add 18 new tests for quota-aware selection

Scoring formula: Health×2 + Tokens×5 + Quota×3 + LRU×0.1

An attempt at resolving  badrisnarayanan/antigravity-claude-proxy#171

src/config.js

@@ -34,6 +34,12 @@ const DEFAULT_CONFIG = {
             maxTokens: 50,            // Maximum token capacity
             tokensPerMinute: 6,       // Regeneration rate
             initialTokens: 50         // Starting tokens
+        },
+        quota: {
+            lowThreshold: 0.10,       // 10% - reduce score
+            criticalThreshold: 0.05,  // 5% - exclude from candidates
+            staleMs: 300000,          // 5 min - max age of quota data to trust
+            weight: 3                 // Scoring weight (same scale as health/tokens)
         }
     }
 };