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/models/__init__.py

@@ -1,5 +1,6 @@
 from .base import Base
 from .users import User
+from .api_tokens import ApiToken
 from .hosts import Host
 from .monitors import Monitor, MonitorResult, MonitorType
 from .metrics import PluginMetric
@@ -11,7 +12,7 @@ from .settings import AppSetting
 from .dashboard import Dashboard, DashboardWidget, DashboardShareToken
 
 __all__ = [
-    "Base", "User",
+    "Base", "User", "ApiToken",
     "Host",
     "Monitor", "MonitorResult", "MonitorType",
     "PluginMetric",