perf(indexes): add critical indexes for Test and AuditLog models (P0)

- Declare __table_args__ on Test with 5 indexes: technique_id, state, created_at, (technique_id,state), (state,created_at)

- Declare __table_args__ on AuditLog with 3 indexes: (entity_type,entity_id), timestamp, (entity_type,entity_id,action)

- Alembic b024: create only the 2 new indexes (ix_tests_created_at, ix_tests_state_created_at); existing indexes from b005/b018/b019 are preserved

- Model index names aligned with existing migration names to prevent duplicates

backend/app/models/audit.py

@@ -1,5 +1,5 @@
 import uuid
-from sqlalchemy import Column, String, DateTime, ForeignKey, func
+from sqlalchemy import Column, String, DateTime, ForeignKey, Index, func
 from sqlalchemy.dialects.postgresql import UUID, JSONB
 from sqlalchemy.orm import relationship
 
@@ -25,3 +25,9 @@ class AuditLog(Base):
 
     # Relationships
     user = relationship("User")
+
+    __table_args__ = (
+        Index("ix_audit_logs_entity", "entity_type", "entity_id"),
+        Index("ix_audit_logs_timestamp", "timestamp"),
+        Index("ix_audit_logs_entity_type_entity_id_action", "entity_type", "entity_id", "action"),
+    )