feat: Phase2影响分析 — EXPLAIN预检+大表检测(简化版)+执行策略+风险评估table_sizes支持 (1141测试)

新增:
- explain_preview.py: EXPLAIN预检服务(MySQL/PG), 影响行数预估, 全表扫描检测
- ticket.executor: 三种执行策略(stop_on_error/continue/transaction)
- risk_evaluator: table_sizes参数, 大表ALTER/UPDATE/DELETE自动升级high
- test_phase2_features.py: 22个测试(EXPLAIN+执行策略+大表检测)

简化:
- 砍掉CTX-001上下文感知规则(复杂二次扫描), 合并大表检测到risk_evaluator
- 审核引擎保持纯静态分析, 不再导入_RULE_REGISTRY做二次扫描

模型:
- Ticket加exec_strategy字段
- TicketSqlItem模型
- schema加sql_type/exec_strategy/risk_level/workflow_id

backend/app/schemas/ticket.py

@@ -13,6 +13,7 @@ class TicketCreate(BaseModel):
     rollback_sql: Optional[str] = None
     reason: str = Field(..., min_length=5)
     priority: Literal["high", "medium", "low"] = "medium"
+    exec_strategy: Optional[str] = "stop_on_error"
     scheduled_at: Optional[datetime] = None