feat(m8): SSE hub + realtime chat with mentions, tests, e2e

- SseHub (project-scoped client mgmt + broadcast, implements SseBroadcaster)
  with SSE-formatted payload; singleton getSseHub
- ChatRepository (message CRUD, soft-delete, search, pagination, project
  isolation) + ChatService (membership permission, send/edit/delete,
  @email mention -> mention notification, SSE broadcast of created/updated/deleted)
- SSE stream endpoint (ReadableStream + heartbeat + abort cleanup, membership)
- Chat message API routes (history/send/edit/delete)
- ChatWindow client (EventSource auto-reconnect, history fetch, realtime
  append/update/delete), chat page, ProjectNav チャット link
- SseEvent type now carries ChatMessage for chat events
- Unit (SseHub, ChatRepository, ChatService) + integration chat-sse-broadcast
  + e2e chat-sse (two contexts realtime)

tests/unit/lib/sse/hub.test.ts

@@ -0,0 +1,92 @@
+import { describe, it, expect, beforeEach } from 'vitest';
+import { SseHub, type SseClient } from '@/lib/sse/hub';
+
+function makeClient(): SseClient & { received: string[] } {
+  const received: string[] = [];
+  return {
+    received,
+    enqueue: (chunk: string) => received.push(chunk),
+    close: () => undefined,
+  };
+}
+
+describe('SseHub', () => {
+  let hub: SseHub;
+
+  beforeEach(() => {
+    hub = new SseHub();
+  });
+
+  it('adds and counts clients per project', () => {
+    hub.addClient(1, makeClient());
+    hub.addClient(1, makeClient());
+    hub.addClient(2, makeClient());
+
+    expect(hub.getClientCount(1)).toBe(2);
+    expect(hub.getClientCount(2)).toBe(1);
+    expect(hub.getClientCount(999)).toBe(0);
+  });
+
+  it('removes a client', () => {
+    const c = makeClient();
+    hub.addClient(1, c);
+    hub.removeClient(1, c);
+    expect(hub.getClientCount(1)).toBe(0);
+  });
+
+  it('broadcasts only to clients of the target project', () => {
+    const a1 = makeClient();
+    const a2 = makeClient();
+    const b1 = makeClient();
+    hub.addClient(1, a1);
+    hub.addClient(1, a2);
+    hub.addClient(2, b1);
+
+    hub.broadcast(1, {
+      type: 'chat.message.created',
+      data: { projectId: 1, message: { id: 1 } as never },
+    });
+
+    expect(a1.received).toHaveLength(1);
+    expect(a2.received).toHaveLength(1);
+    expect(b1.received).toHaveLength(0);
+    expect(a1.received[0]).toContain('chat.message.created');
+  });
+
+  it('broadcast payload is SSE formatted (data: ...\\n\\n)', () => {
+    const c = makeClient();
+    hub.addClient(1, c);
+    hub.broadcast(1, {
+      type: 'chat.message.deleted',
+      data: { projectId: 1, id: 5 },
+    });
+
+    expect(c.received[0]).toMatch(/^data: .+\n\n$/);
+    const parsed = JSON.parse(
+      c.received[0].replace(/^data: /, '').replace(/\n\n$/, '')
+    );
+    expect(parsed.type).toBe('chat.message.deleted');
+    expect(parsed.data.id).toBe(5);
+  });
+
+  it('does nothing when there are no clients for the project', () => {
+    expect(() =>
+      hub.broadcast(42, {
+        type: 'note.updated',
+        data: { projectId: 42 },
+      })
+    ).not.toThrow();
+  });
+
+  it('removes a client that throws on enqueue', () => {
+    const broken: SseClient = {
+      enqueue: () => {
+        throw new Error('disconnected');
+      },
+      close: () => undefined,
+    };
+    hub.addClient(1, broken);
+    hub.broadcast(1, { type: 'note.updated', data: { projectId: 1 } });
+    expect(hub.getClientCount(1)).toBe(0);
+  });
+});