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.

web/src/api/game.ts

@@ -66,6 +66,34 @@ export interface LLMConfigDTO {
   mode: string;
 }
 
+// --- Events Pagination ---
+
+export interface EventDTO {
+  id: string;
+  text: string;
+  content: string;
+  year: number;
+  month: number;
+  month_stamp: number;
+  related_avatar_ids: string[];
+  is_major: boolean;
+  is_story: boolean;
+}
+
+export interface EventsResponseDTO {
+  events: EventDTO[];
+  next_cursor: string | null;
+  has_more: boolean;
+}
+
+export interface FetchEventsParams {
+  avatar_id?: string;
+  avatar_id_1?: string;
+  avatar_id_2?: string;
+  cursor?: string;
+  limit?: number;
+}
+
 export const gameApi = {
   // --- World State ---
   
@@ -165,5 +193,25 @@ export const gameApi = {
 
   saveLLMConfig(config: LLMConfigDTO) {
     return httpClient.post<{ status: string; message: string }>('/api/config/llm/save', config);
+  },
+
+  // --- Events Pagination ---
+
+  fetchEvents(params: FetchEventsParams = {}) {
+    const query = new URLSearchParams();
+    if (params.avatar_id) query.set('avatar_id', params.avatar_id);
+    if (params.avatar_id_1) query.set('avatar_id_1', params.avatar_id_1);
+    if (params.avatar_id_2) query.set('avatar_id_2', params.avatar_id_2);
+    if (params.cursor) query.set('cursor', params.cursor);
+    if (params.limit) query.set('limit', String(params.limit));
+    const qs = query.toString();
+    return httpClient.get<EventsResponseDTO>(`/api/events${qs ? '?' + qs : ''}`);
+  },
+
+  cleanupEvents(keepMajor = true, beforeMonthStamp?: number) {
+    const query = new URLSearchParams();
+    query.set('keep_major', String(keepMajor));
+    if (beforeMonthStamp !== undefined) query.set('before_month_stamp', String(beforeMonthStamp));
+    return httpClient.delete<{ deleted: number }>(`/api/events/cleanup?${query}`);
   }
 };