fix: reduce false container-failed notifications, add error details to UI

Backend (health_monitor.py):
- Skip health checks for instances with no container_id
- Treat 'not_found' as error only when container was previously running
- Skip duplicate error notifications when already in error state
- Skip 'not_found' notifications for containers that never ran

Frontend (notification-item.tsx):
- Display notification.message (detailed error text)
- Add expandable Details section showing metadata (exit_code, previous_status, etc.)
- New CSS styles for message and metadata display

Quality gates: py_compile, tsc --noEmit, 80/80 tests pass

apps/web/src/styles/utilities.css

@@ -2474,6 +2474,44 @@ a:active,
 	color: var(--muted);
 }
 
+.notification-item-message {
+	font-size: 0.85rem;
+	color: var(--muted);
+	line-height: 1.35;
+	margin-top: 0.15rem;
+	word-break: break-word;
+}
+
+.notification-item-metadata {
+	margin-top: 0.5rem;
+	padding: 0.5rem 0.75rem;
+	background: var(--bg);
+	border-radius: 6px;
+	border: 1px solid var(--border);
+	font-size: 0.8rem;
+}
+
+.notification-metadata-row {
+	display: flex;
+	gap: 0.5rem;
+	padding: 0.15rem 0;
+}
+
+.notification-metadata-row dt {
+	font-weight: 500;
+	color: var(--ink);
+	min-width: 6rem;
+	text-transform: capitalize;
+	flex-shrink: 0;
+}
+
+.notification-metadata-row dd {
+	margin: 0;
+	color: var(--muted);
+	font-family: monospace;
+	word-break: break-word;
+}
+
 .notification-item-actions {
 	display: flex;
 	gap: 0.35rem;