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/app.py

@@ -150,6 +150,7 @@ def create_app(
     from .ansible.inventory_routes import inventory_bp
     from .settings.routes import settings_bp
     from .audit.routes import audit_bp
+    from .ai_access.routes import ai_access_bp
 
     app.register_blueprint(auth_bp)
     app.register_blueprint(dashboard_bp)
@@ -161,6 +162,7 @@ def create_app(
     app.register_blueprint(inventory_bp)
     app.register_blueprint(settings_bp)
     app.register_blueprint(audit_bp)
+    app.register_blueprint(ai_access_bp)
 
     # Register the unified Monitor status source for the Status page. Plugins
     # may register additional sources from setup(). Idempotent.