feat(tasks): add optional "none" priority level for tasks without urgency

New tasks default to "none" priority instead of "medium". Tasks with no
priority hide the badge in list and dashboard views, reducing visual noise
for routine items. Includes DB migration v4 and i18n keys (de, en, it).

Closes #15

server/routes/tasks.js

@@ -18,7 +18,7 @@ const router = express.Router();
 // Konstanten
 // --------------------------------------------------------
 
-const VALID_PRIORITIES = ['low', 'medium', 'high', 'urgent'];
+const VALID_PRIORITIES = ['none', 'low', 'medium', 'high', 'urgent'];
 const VALID_STATUSES   = ['open', 'in_progress', 'done'];
 const VALID_CATEGORIES = ['Haushalt', 'Schule', 'Einkauf', 'Reparatur',
                           'Gesundheit', 'Finanzen', 'Freizeit', 'Sonstiges'];
@@ -96,7 +96,7 @@ router.get('/', (req, res) => {
       ORDER BY
         CASE t.status WHEN 'done' THEN 1 ELSE 0 END,
         CASE t.priority WHEN 'urgent' THEN 0 WHEN 'high' THEN 1
-                        WHEN 'medium' THEN 2 ELSE 3 END,
+                        WHEN 'medium' THEN 2 WHEN 'low' THEN 3 ELSE 4 END,
         t.due_date ASC NULLS LAST,
         t.created_at DESC
     `;
@@ -148,7 +148,7 @@ router.post('/', (req, res) => {
       title,
       description     = null,
       category        = 'Sonstiges',
-      priority        = 'medium',
+      priority        = 'none',
       due_date        = null,
       due_time        = null,
       assigned_to     = null,