add dynamic profile update, connection helper, file download metadata

- replace UserSetColor/UserSetPronouns with single UserUpdateProfile that dynamically builds one UPDATE query from UserProfileUpdateList
- add getConnectionWithResponseOnFail helper to deduplicate connection ID parsing and validation across handlers
- rename file.go to attachmentFile.go and update handler names
- GetDownloadUrlAndMetadata now fetches object metadata via StatObject and returns it alongside the presigned URL
- file download endpoint returns JSON with url and originalName
- add description field to user and DB schema
- remove unused ConnectionState variants (GroupFellow, GroupFriend)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/types/types.go

@@ -28,6 +28,9 @@ type User struct {
 	Mu           sync.RWMutex
 	Name         string
 	Pronouns     string
+	Description  string
+	Avatar       string
+	ProfileBg    string
 	PasswordHash string
 	CreatedAt    time.Time
 	WsConn       *websocket.Conn
@@ -36,6 +39,12 @@ type User struct {
 	Color        *Rgba
 }
 
+type UserProfileUpdateList struct {
+	Pronouns    bool
+	Description bool
+	Color       bool
+}
+
 type Connection struct {
 	Mu                   sync.RWMutex                        `json:"-"`
 	Id                   uuid.UUID                           `json:"id"`