Implement newsletter scheduling features

- Added a background scheduler to manage newsletter dispatch based on user-defined settings.
- Enhanced the SMTP settings to include scheduling options such as frequency, time, and specific weekdays.
- Updated the dashboard UI to allow users to configure scheduling preferences for automated newsletter sending.
- Modified the newsletter generation logic to accommodate new scheduling parameters and ensure proper content delivery.

app/static/css/main.css

@@ -707,6 +707,39 @@ body.login-page .container {
     flex-wrap: wrap;
 }
 
+/* ── Zeitplan ── */
+.schedule-box {
+    border: 1px solid var(--tk-border);
+    border-radius: 8px;
+    padding: 0.85rem 1rem 1rem;
+    margin: 0.25rem 0;
+}
+
+.schedule-box legend {
+    font-weight: 600;
+    padding: 0 0.4rem;
+    color: var(--tk-orange-dark, #c15200);
+}
+
+.weekday-row {
+    display: flex;
+    gap: 0.4rem;
+    flex-wrap: wrap;
+    margin: 0.25rem 0 0.75rem;
+}
+
+.weekday-chip {
+    display: inline-flex;
+    align-items: center;
+    gap: 0.3rem;
+    border: 1px solid var(--tk-border);
+    border-radius: 6px;
+    padding: 0.3rem 0.55rem;
+    font-size: 0.85rem;
+    cursor: pointer;
+    user-select: none;
+}
+
 /* ── Footer ── */
 .site-footer {
     text-align: center;