fix: guide users to Auto-Tmux when a persisted session expires (#1336)

* fix: guide users to Auto-Tmux when a persisted session expires

A timed-out terminal session silently reconnected to a fresh shell, so
people running long jobs lost them with no explanation and never learned
about Auto-Tmux. Explain the expiry with a one-click Enable Auto-Tmux
action, let admins default it for new hosts and tune the persistence
timeout from the UI, and move the setting up with copy that says what it
does. The global default stays off.

* style: format terminal expiry notice

src/backend/database/routes/host.ts

@@ -1459,6 +1459,90 @@ router.put(
  *       500:
  *         description: Failed to fetch SSH data.
  */
+/**
+ * @openapi
+ * /host/db/host/{id}/terminal-config:
+ *   patch:
+ *     summary: Update a host's terminal behaviour flags
+ *     description: Merges the given flags into the host's terminalConfig. Owner or a recipient with edit access. Currently supports autoTmux; used by the "enable Auto-Tmux" action shown when a persisted session expires.
+ *     tags:
+ *       - Hosts
+ *     requestBody:
+ *       required: true
+ *       content:
+ *         application/json:
+ *           schema:
+ *             type: object
+ *             properties:
+ *               autoTmux:
+ *                 type: boolean
+ *     responses:
+ *       200:
+ *         description: Updated.
+ *       403:
+ *         description: No edit access.
+ *       404:
+ *         description: Host not found.
+ */
+router.patch(
+  "/db/host/:id/terminal-config",
+  authenticateJWT,
+  permissionManager.requirePermission("hosts.edit"),
+  requireDataAccess,
+  async (req: Request, res: Response) => {
+    const userId = (req as AuthenticatedRequest).userId!;
+    const hostId = parseInt(String(req.params.id), 10);
+    const { autoTmux } = req.body ?? {};
+
+    if (isNaN(hostId)) {
+      return res.status(400).json({ error: "Invalid host ID" });
+    }
+    if (typeof autoTmux !== "boolean") {
+      return res.status(400).json({ error: "autoTmux must be a boolean" });
+    }
+
+    try {
+      const access = await permissionManager.canAccessHost(
+        userId,
+        hostId,
+        "edit",
+      );
+      if (!access.hasAccess) {
+        return res.status(403).json({ error: "Access denied to host" });
+      }
+      const ownerId =
+        await createCurrentHostResolutionRepository().findHostOwnerId(hostId);
+      const host = ownerId
+        ? await createCurrentHostRepository().findByIdForUser(ownerId, hostId)
+        : null;
+      if (!host || !ownerId) {
+        return res.status(404).json({ error: "Host not found" });
+      }
+
+      let terminalConfig: Record<string, unknown> = {};
+      if (host.terminalConfig) {
+        try {
+          terminalConfig = JSON.parse(host.terminalConfig);
+        } catch {
+          terminalConfig = {};
+        }
+      }
+      await createCurrentHostRepository().updateForUser(ownerId, hostId, {
+        terminalConfig: JSON.stringify({ ...terminalConfig, autoTmux }),
+      });
+
+      res.json({ success: true, autoTmux });
+    } catch (error) {
+      sshLogger.error("Failed to update host terminal config", error, {
+        operation: "host_terminal_config_update",
+        hostId,
+        userId,
+      });
+      res.status(500).json({ error: "Failed to update terminal config" });
+    }
+  },
+);
+
 router.get(
   "/db/host",
   authenticateJWT,