Update message_cache.go

Added lock in add_messages to avoid "database is locked" error
Small code reformatting

server/message_cache.go

@@ -8,6 +8,7 @@ import (
 	"net/netip"
 	"strings"
 	"time"
+	"sync"
 
 	_ "github.com/mattn/go-sqlite3" // SQLite driver
 	"heckel.io/ntfy/v2/log"
@@ -105,7 +106,7 @@ const (
 		WHERE topic = ? AND published = 1
 		ORDER BY time DESC, id DESC
 		LIMIT 1
-  `
+	`
 	selectMessagesDueQuery = `
 		SELECT mid, time, expires, topic, message, title, priority, tags, click, icon, actions, attachment_name, attachment_type, attachment_size, attachment_expires, attachment_url, sender, user, content_type, encoding
 		FROM messages
@@ -281,6 +282,7 @@ var (
 type messageCache struct {
 	db    *sql.DB
 	queue *util.BatchingQueue[*message]
+	mu    sync.Mutex
 	nop   bool
 }
 
@@ -340,6 +342,8 @@ func (c *messageCache) AddMessage(m *message) error {
 // addMessages synchronously stores a match of messages. If the database is locked, the transaction waits until
 // SQLite's busy_timeout is exceeded before erroring out.
 func (c *messageCache) addMessages(ms []*message) error {
+	c.mu.Lock()
+	defer c.mu.Unlock()
 	if c.nop {
 		return nil
 	}