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",

steward/models/api_tokens.py

@@ -0,0 +1,35 @@
+from __future__ import annotations
+from datetime import datetime, timezone
+from sqlalchemy import DateTime, Enum, ForeignKey, Integer, String
+from sqlalchemy.orm import Mapped, mapped_column
+from .base import Base
+from .users import UserRole
+
+
+class ApiToken(Base):
+    """A personal bearer token an AI client (MCP) uses to act as its user.
+
+    Only the sha256 of the token is stored; the plaintext is shown once at
+    creation. `role` is a ceiling: the token acts with the lower of this and
+    its user's current role, so demoting a user demotes their tokens too.
+    """
+    __tablename__ = "api_tokens"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+    user_id: Mapped[str] = mapped_column(
+        String(36), ForeignKey("users.id", ondelete="CASCADE"), nullable=False,
+    )
+    name: Mapped[str] = mapped_column(String(128), nullable=False)
+    token_hash: Mapped[str] = mapped_column(String(64), nullable=False, unique=True)
+    # Leading characters of the plaintext, for telling tokens apart in the UI.
+    prefix: Mapped[str] = mapped_column(String(16), nullable=False)
+    role: Mapped[UserRole] = mapped_column(
+        Enum(UserRole, name="userrole", create_type=False), nullable=False,
+    )
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False,
+        default=lambda: datetime.now(timezone.utc),
+    )
+    last_used_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
+    expires_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
+    revoked_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)