fix: replace sql.NullTime with string scan for MAX() aggregate queries

Go 1.26 changed NullTime.Scan to delegate to convertAssign,
which has no string->time.Time conversion path. The
modernc.org/sqlite driver returns raw strings for aggregate
expressions like MAX(last_used_at), causing silent scan failures
that made all clients/providers show 'Never' for last used.

Fixes by scanning into a string and parsing with time.Parse.

internal/server/clients.go

@@ -1,7 +1,6 @@
 package server
 
 import (
-	"database/sql"
 	"net/http"
 	"time"
 
@@ -47,13 +46,14 @@ func (s *Server) handleGetClients(c *gin.Context) {
 			desc = *cl.Description
 		}
 
-		var lastUsedTime sql.NullTime
-		_ = s.database.Get(&lastUsedTime, "SELECT MAX(last_used_at) FROM client_tokens WHERE client_id = ?", cl.ClientID)
+		var lastUsedStr string
+		_ = s.database.Get(&lastUsedStr, "SELECT MAX(last_used_at) FROM client_tokens WHERE client_id = ?", cl.ClientID)
 
 		var lastUsed *time.Time
-		if lastUsedTime.Valid && !lastUsedTime.Time.IsZero() {
-			t := lastUsedTime.Time
-			lastUsed = &t
+		if lastUsedStr != "" {
+			if t, err := time.Parse("2006-01-02 15:04:05", lastUsedStr); err == nil {
+				lastUsed = &t
+			}
 		}
 
 		uiClients[i] = UIClient{