feat(engine): 实现阵营投票系统

- 新增 Votes 模型，包含 optimists/fearful 计数和 voted_users 去重列表
- 扩展 Event 模型，添加可选 faction 字段支持投票事件
- 新增 voting.py 模块处理投票逻辑
- 投票规则：每用户每 tick 限投一次，下一 tick 生效
- 投票累加到 factions.power，达到 threshold 触发阵营技能
- 添加 5 个投票系统测试用例，全部 26 个测试通过

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

engine-python/app/models.py

@@ -58,6 +58,14 @@ class Factions(BaseModel):
     fearful: FactionData = Field(default_factory=FactionData)
 
 
+class Votes(BaseModel):
+    """投票统计"""
+    optimists: int = 0
+    fearful: int = 0
+    # 记录本 tick 已投票的用户（用于去重）
+    voted_users: List[str] = Field(default_factory=list)
+
+
 class FactionEventResult(BaseModel):
     """阵营事件触发结果"""
     type: Optional[str] = None  # "festival" | "panic" | None
@@ -104,6 +112,7 @@ class WorldState(BaseModel):
     world_effects: List[WorldEffect] = Field(default_factory=list)
     factions: Factions = Field(default_factory=Factions)
     story_arcs: Dict[str, StoryArc] = Field(default_factory=dict)
+    votes: Votes = Field(default_factory=Votes)
 
 
 class Event(BaseModel):
@@ -111,6 +120,7 @@ class Event(BaseModel):
     text: str
     user: str
     ts: float
+    faction: Optional[str] = None  # 用于投票事件: "optimists" | "fearful"
 
 
 class StepRequest(BaseModel):