Fix code review feedback items 1-6, 8-10

- Item 1: Extract GetUserByID/GetChannelByID lookup methods, use from relation methods
- Item 2: Initialize slices with literals so JSON gets [] not null
- Item 3: Populate CreatedAt/UpdatedAt with time.Now() on all Create methods
- Item 4: Wrap each migration's SQL + recording in a transaction
- Item 5: Check error from res.LastInsertId() in QueueMessage
- Item 6: Add DequeueMessages and AckMessages methods
- Item 8: Add GetUserByNick, GetUserByToken, DeleteAuthToken, UpdateUserLastSeen
- Item 9: Run PRAGMA foreign_keys = ON on every new connection
- Item 10: Builds clean, all tests pass

internal/models/user.go

@@ -33,7 +33,7 @@ func (u *User) Channels(ctx context.Context) ([]*Channel, error) {
 
 	defer func() { _ = rows.Close() }()
 
-	var channels []*Channel
+	channels := []*Channel{}
 
 	for rows.Next() {
 		c := &Channel{}
@@ -70,7 +70,7 @@ func (u *User) QueuedMessages(ctx context.Context) ([]*Message, error) {
 
 	defer func() { _ = rows.Close() }()
 
-	var messages []*Message
+	messages := []*Message{}
 
 	for rows.Next() {
 		msg := &Message{}