feat: filter notifications to warnings/errors/ready only and add clear-all button

Notification filtering:
- lifecycle_hooks.py: only instance.error and instance.health_changed
  with status=running generate notifications. All other lifecycle events
  (created, started, stopped, restarted, deleted) are filtered out.
- health_monitor.py: only error and unhealthy states generate notifications.
  Running/recovered state no longer creates info notifications.
- _derive_title now maps instance.health_changed to "Container ready".

Clear-all button:
- Added dismiss_all() to NotificationService
- Added DELETE /notifications endpoint for bulk dismiss
- Frontend: clearAllNotifications API, clearAll in notification context,
  "Clear all" button in notification drawer alongside "Mark all as read"
- Added CSS for .notification-clear-all with danger hover state
- Updated notification-center tests

Quality gates: pytest (21 passed), vitest (11 passed)

apps/web/src/styles.css

@@ -4602,10 +4602,12 @@ a:active,
 	padding: 0.75rem 1rem;
 	border-top: 1px solid var(--border);
 	flex-shrink: 0;
+	display: flex;
+	gap: 0.5rem;
 }
 
 .notification-mark-all {
-	width: 100%;
+	flex: 1;
 	padding: 0.5rem 0.75rem;
 	background: transparent;
 	border: 1px solid var(--border);
@@ -4623,6 +4625,25 @@ a:active,
 	border-color: var(--brand);
 }
 
+.notification-clear-all {
+	flex: 1;
+	padding: 0.5rem 0.75rem;
+	background: transparent;
+	border: 1px solid var(--border);
+	border-radius: 8px;
+	color: var(--muted);
+	font: inherit;
+	font-size: 0.85rem;
+	cursor: pointer;
+	transition: all 0.15s ease;
+}
+
+.notification-clear-all:hover {
+	background: var(--bg);
+	color: var(--danger);
+	border-color: var(--danger);
+}
+
 /* Notification Item */
 .notification-item {
 	display: flex;