feat(api): personal API tokens and the AI access page

First step of milestone 461 (MCP server). A token is the credential an
MCP client presents as `Authorization: Bearer stw_…` to act as its user.

- api_tokens table (0026): sha256 hash only, display prefix, role
  ceiling, expiry, revocation, throttled last_used_at.
- steward/core/api_tokens.py: mint, list, revoke, resolve. Resolution
  re-reads the user's current role and acts with the lower of that and
  the token's ceiling, so demoting or disabling a user weakens or kills
  their tokens with nothing to revoke by hand.
- /ai-access/ (sidebar → Automation → AI access): every user manages
  their own tokens, can't grant above their own role, sees the token
  once. Session-only: a share-link viewer is refused. Create and revoke
  are audited.
- Integration tests drive the routes against real Postgres so their
  transactions run for real (#884 escaped CI behind a mocked session).

Task #4965 "MCP-1 — Personal API tokens".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

steward/templates/base.html

@@ -313,6 +313,7 @@ body.dash-editing .widget-drawer { transform:translateY(0); }
       <div class="nav-group">
         <div class="nav-group-label">Automation</div>
         <a href="/ansible/" {% if _p.startswith('/ansible') %}class="active" aria-current="page"{% endif %}>Ansible</a>
+        <a href="/ai-access/" {% if _p.startswith('/ai-access') %}class="active" aria-current="page"{% endif %}>AI access</a>
       </div>
       {% if session.user_role == 'admin' %}
       <div class="nav-group">