feat(ansible): host provisioning via steward managed SSH identity

Turn the agent-install playbook into a full provisioning + maintenance
path. Solves the bootstrap chicken-and-egg: first contact uses an
operator-supplied password (one run, never stored), which creates a
dedicated `steward` login account with NOPASSWD sudo + Steward's managed
public key. Every run thereafter connects as `steward` with the managed
key — fully unattended (scheduled prune, agent updates).

- core/crypto: generate_ssh_keypair() — ed25519, OpenSSH formats.
- settings: ansible.ssh_public_key (non-secret, displayed) + ansible.ssh_user
  (default steward); to_ansible_cfg extended.
- settings UI + route: "Generate managed key" (private encrypted, public
  shown to copy) + SSH-user field.
- executor: build_bootstrap() writes a 0600 vars file (-e @file) for the
  per-run user/password — never argv, never DB, never logged; drops the
  managed key when a bootstrap password is given; --user floor from the
  global ssh_user when no override.
- runner.trigger_run: pass-through `connection` kwarg, deliberately NOT
  persisted on AnsibleRun.params (password stays out of the DB).
- bundled/host_agent/provision.yml: create steward user + authorized_keys
  + /etc/sudoers.d/steward (visudo-validated) + agent install.
- host_agent: /provision route + "Provision a fresh host" card (bootstrap
  user/password; injects pubkey + user + token as space-safe JSON hostvars).
- Dockerfile: add sshpass (Ansible shells out to it for password SSH).
- tests: keypair generation + build_bootstrap (secret stays off argv).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

steward/ansible/runner.py

@@ -25,6 +25,7 @@ async def trigger_run(
     params: dict | None = None,
     triggered_by: str | None = None,
     inventory_content: str | None = None,
+    connection: dict | None = None,
 ):
     """Resolve inventory for the scope, persist an AnsibleRun, and launch it.
 
@@ -32,6 +33,9 @@ async def trigger_run(
     If inventory_content is provided, it is used verbatim and scope resolution
     is skipped (the caller built a bespoke inventory — e.g. host_agent deploy
     injecting per-host tokens); inventory_scope is still recorded for display.
+    connection is an optional per-run SSH override (user/password) for
+    first-contact provisioning — passed to the executor but deliberately NOT
+    stored on the AnsibleRun row (params), so the password never lands in the DB.
     Returns (run, source, error): on success error is None; on failure run is
     None and error is a short human-readable reason.
     """
@@ -73,6 +77,7 @@ async def trigger_run(
         executor.start_run(
             app, run_id, playbook_path, inventory_path or "",
             source["path"], params or None, inventory_content,
+            connection=connection,
         )
     )
     task.add_done_callback(