feat(server): add configurable SSH connection timeout per server

Add `connection_timeout` field to server settings, allowing per-server
override of the global SSH connection timeout constant.

- Migration adds `connection_timeout` integer column (default 10s)
- `ServerSetting` model exposes and casts the new field
- `SshMultiplexingHelper::getConnectionTimeout()` resolves per-server
  value with fallback to `constants.ssh.connection_timeout`
- All SSH/SCP command builders use the new resolver instead of the
  global config directly
- Livewire `Show` component binds `connectionTimeout` with validation
  (1–300 seconds) and syncs to/from the model
- UI input added to server settings form with helper text
- Feature tests cover default, persistence, resolver, and fallback

openapi.json

@@ -13349,6 +13349,10 @@
                     "delete_unused_networks": {
                         "type": "boolean",
                         "description": "The flag to indicate if the unused networks should be deleted."
+                    },
+                    "connection_timeout": {
+                        "type": "integer",
+                        "description": "SSH connection timeout in seconds."
                     }
                 },
                 "type": "object"