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.yaml

@@ -8538,6 +8538,9 @@ components:
         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
     Service:
       description: 'Service model'