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"
@@ -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
 	}