feat: 后端架构重构 — Endpoint→Service→Repository分层 + AI确认机制 + 异步任务持久化

- 核心业务拆分为 Endpoint → Application Service → Repository 三层
- AI写入操作必须用户确认后才写库(确认卡片机制)
- 报告/饮食/用药分析改为持久化任务队列(原子领取/重试/重启恢复)
- 运动计划修复: 连续真实日期替代周模板
- 用药提醒去重 + 通知Outbox预留
- 认证收拢到AuthService, 管理员收拢到AdminService
- AI会话加用户归属校验防串号
- 提示词调整为患者视角
- 开发假数据已关闭
- 21/21测试通过, 0警告0错误

backend/src/Health.Infrastructure/AI/EfAiConversationRepository.cs

@@ -0,0 +1,45 @@
+using Health.Application.AI;
+
+namespace Health.Infrastructure.AI;
+
+public sealed class EfAiConversationRepository(AppDbContext db) : IAiConversationRepository
+{
+    private readonly AppDbContext _db = db;
+
+    public Task<Conversation?> GetOwnedAsync(Guid userId, Guid conversationId, CancellationToken ct) =>
+        _db.Conversations.FirstOrDefaultAsync(c => c.Id == conversationId && c.UserId == userId, ct);
+
+    public Task<Conversation?> GetAsync(Guid conversationId, CancellationToken ct) =>
+        _db.Conversations.FirstOrDefaultAsync(c => c.Id == conversationId, ct);
+
+    public async Task<IReadOnlyList<Conversation>> ListAsync(Guid userId, CancellationToken ct) =>
+        await _db.Conversations
+            .Where(c => c.UserId == userId)
+            .OrderByDescending(c => c.UpdatedAt)
+            .ToListAsync(ct);
+
+    public async Task<IReadOnlyList<ConversationMessage>> GetMessagesAsync(Guid conversationId, CancellationToken ct) =>
+        await _db.ConversationMessages
+            .Where(m => m.ConversationId == conversationId)
+            .OrderBy(m => m.CreatedAt)
+            .ToListAsync(ct);
+
+    public async Task<IReadOnlyList<ConversationMessage>> GetRecentMessagesAsync(Guid conversationId, int limit, CancellationToken ct) =>
+        await _db.ConversationMessages
+            .Where(m => m.ConversationId == conversationId)
+            .OrderByDescending(m => m.CreatedAt)
+            .Take(limit)
+            .ToListAsync(ct);
+
+    public async Task AddConversationAsync(Conversation conversation, CancellationToken ct) =>
+        await _db.Conversations.AddAsync(conversation, ct);
+
+    public async Task AddMessageAsync(ConversationMessage message, CancellationToken ct) =>
+        await _db.ConversationMessages.AddAsync(message, ct);
+
+    public void Delete(Conversation conversation) =>
+        _db.Conversations.Remove(conversation);
+
+    public Task SaveChangesAsync(CancellationToken ct) =>
+        _db.SaveChangesAsync(ct);
+}