feat: SQLite event storage with pagination and filtering

Implement SQLite-based event persistence as specified in sqlite-event-manager.md.

## Changes

### Backend
- **EventStorage** (`src/classes/event_storage.py`): New SQLite storage layer
  - Cursor-based pagination with compound cursor `{month_stamp}_{rowid}`
  - Avatar filtering (single and pair queries)
  - Major/minor event separation
  - Cleanup API with `keep_major` and `before_month_stamp` filters

- **EventManager** (`src/classes/event_manager.py`): Refactored to use SQLite
  - Delegates to EventStorage for persistence
  - Memory fallback mode for testing
  - New `get_events_paginated()` method

- **API** (`src/server/main.py`):
  - `GET /api/events` - Paginated event retrieval with filtering
  - `DELETE /api/events/cleanup` - User-triggered cleanup

### Frontend
- **EventPanel.vue**: Scroll-to-load pagination, dual-person filter UI
- **world.ts**: Event state management with pagination
- **game.ts**: New API client methods

### Testing
- 81 new tests for EventStorage, EventManager, and API
- Added `pytest-asyncio` and `httpx` to requirements.txt

## Known Issues: Save/Load is Currently Broken

After loading a saved game, the following issues occur:

1. **Wrong database used**: API returns events from the startup database instead
   of the loaded save's `_events.db` file
2. **Events from wrong time period**: Shows events from year 115 when loaded
   save is at year 114
3. **Pagination broken after load**: `has_more` returns `False` despite hundreds
   of events in the saved database
4. **Filter functionality broken**: Character selection filter stops working
   after loading a game

Root cause: `load_game.py` does not properly switch the EventManager's database
connection to the loaded save's events database.

src/classes/world.py

@@ -1,4 +1,5 @@
 from dataclasses import dataclass, field
+from pathlib import Path
 from typing import TYPE_CHECKING, Optional
 
 from src.classes.map import Map
@@ -63,4 +64,29 @@ class World():
             "动作": "你有一系列可以执行的动作。要注意动作的效果、限制条件、区域和时间。",
             "装备与丹药": "通过兵器、辅助装备、丹药等装备，可以获得额外的属性加成，获得或小或大的增益。拥有好的装备或者服用好的丹药，能获得很大好处。",
         }
-        return desc
+        return desc
+
+    @classmethod
+    def create_with_db(
+        cls,
+        map: "Map",
+        month_stamp: MonthStamp,
+        events_db_path: Path,
+    ) -> "World":
+        """
+        工厂方法：创建使用 SQLite 持久化事件的 World 实例。
+
+        Args:
+            map: 地图对象。
+            month_stamp: 时间戳。
+            events_db_path: 事件数据库文件路径。
+
+        Returns:
+            配置好的 World 实例。
+        """
+        event_manager = EventManager.create_with_db(events_db_path)
+        return cls(
+            map=map,
+            month_stamp=month_stamp,
+            event_manager=event_manager,
+        )