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/test.py

@@ -1,5 +1,5 @@
 import uuid
-from sqlalchemy import Column, String, Text, Boolean, Integer, DateTime, ForeignKey, Enum, func
+from sqlalchemy import Column, String, Text, Boolean, Integer, DateTime, ForeignKey, Enum, Index, func
 from sqlalchemy.dialects.postgresql import UUID
 from sqlalchemy.orm import relationship
 
@@ -72,3 +72,11 @@ class Test(Base):
     remediation_user = relationship("User", foreign_keys=[remediation_assignee])
     original_test = relationship("Test", remote_side="Test.id", foreign_keys=[retest_of])
     retests = relationship("Test", foreign_keys=[retest_of], back_populates="original_test")
+
+    __table_args__ = (
+        Index("ix_tests_technique_id", "technique_id"),
+        Index("ix_tests_state", "state"),
+        Index("ix_tests_created_at", "created_at"),
+        Index("ix_tests_technique_state", "technique_id", "state"),
+        Index("ix_tests_state_created_at", "state", "created_at"),
+    )