feat(server): split file storage into a separate file service with tickets, receipts and WebDAV backup

- new projects WebDrop.FileApi (file service), WebDrop.FileProtocol (shared tickets and receipts) and WebDrop.Sync (WebDAV)
- file content of any size goes to the file service; chat, avatars and space structure stay on the main backend
- tickets issued by the main backend, receipts by the file service, deletion queue and daily reconciliation
- daily database snapshot and file sync to external storage (OpenList/WebDAV) with a 30 day retention
- reorganize Core, Storage and Api into feature folders (namespaces unchanged)
- web client uploads file frames to the file service and reads them with tickets

server/src/WebDrop.Storage/Sqlite/SqliteAccountProfiles.cs

@@ -0,0 +1,38 @@
+using WebDrop.Core;
+
+namespace WebDrop.Storage;
+
+public sealed partial class SqliteAccountStore
+{
+    public AccountProfile Profile(string username)
+    {
+        lock (_gate)
+        {
+            using var cmd = _connection.CreateCommand();
+            cmd.CommandText = "SELECT display_name,avatar,modified_at,avatar_thumb FROM account_profiles WHERE username=$u";
+            cmd.Parameters.AddWithValue("$u", username);
+            using var reader = cmd.ExecuteReader();
+            return reader.Read() ? new AccountProfile { Username = username, DisplayName = reader.GetString(0), Avatar = reader.IsDBNull(1) ? null : reader.GetString(1), HasAvatar = !reader.IsDBNull(1), ModifiedAt = reader.GetInt64(2), AvatarThumb = reader.IsDBNull(3) ? null : reader.GetString(3) }
+                : new AccountProfile { Username = username, DisplayName = username, ModifiedAt = 0 };
+        }
+    }
+    public AccountProfile UpdateProfile(string username, string displayName, string? avatar, string? avatarThumb = null, bool keepAvatar = false)
+    {
+        if (displayName is null || displayName.Trim().Length > 48 || displayName.Any(char.IsControl)) throw new WebDropException(ErrorCode.BadRequest);
+        if (keepAvatar) { var current = Profile(username); avatar = current.Avatar; avatarThumb = current.AvatarThumb; }
+        else if (avatar is not null) AvatarData.Validate(avatar, AvatarData.MaxAccountGifChars);
+        if (!keepAvatar && avatarThumb is not null) { if (avatar is null) throw new WebDropException(ErrorCode.BadRequest); AvatarData.ValidateThumb(avatarThumb); }
+        lock (_gate)
+        {
+            using var cmd = _connection.CreateCommand();
+            cmd.CommandText = "INSERT INTO account_profiles(username,display_name,avatar,avatar_thumb,modified_at) VALUES($u,$n,$a,$b,$t) ON CONFLICT(username) DO UPDATE SET display_name=excluded.display_name,avatar=excluded.avatar,avatar_thumb=excluded.avatar_thumb,modified_at=MAX(account_profiles.modified_at+1,excluded.modified_at)";
+            cmd.Parameters.AddWithValue("$u", username);
+            cmd.Parameters.AddWithValue("$n", displayName.Trim());
+            cmd.Parameters.AddWithValue("$a", (object?)avatar ?? DBNull.Value);
+            cmd.Parameters.AddWithValue("$b", (object?)avatarThumb ?? DBNull.Value);
+            cmd.Parameters.AddWithValue("$t", DateTimeOffset.UtcNow.ToUnixTimeMilliseconds());
+            cmd.ExecuteNonQuery();
+            return Profile(username);
+        }
+    }
+}