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/migrations/versions/0026_api_tokens.py

@@ -0,0 +1,41 @@
+"""Personal API tokens for MCP clients
+
+Revision ID: 0026_api_tokens
+Revises: 0025_drop_http_plugin_setting
+Create Date: 2026-10-06
+"""
+from typing import Sequence, Union
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+revision: str = "0026_api_tokens"
+down_revision: Union[str, None] = "0025_drop_http_plugin_setting"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "api_tokens",
+        sa.Column("id", sa.Integer, primary_key=True, autoincrement=True),
+        sa.Column("user_id", sa.String(36),
+                  sa.ForeignKey("users.id", ondelete="CASCADE"), nullable=False),
+        sa.Column("name", sa.String(128), nullable=False),
+        sa.Column("token_hash", sa.String(64), nullable=False, unique=True),
+        sa.Column("prefix", sa.String(16), nullable=False),
+        # Reuses the users.role enum type created in 0001.
+        sa.Column("role", postgresql.ENUM("admin", "operator", "viewer",
+                                          name="userrole", create_type=False),
+                  nullable=False),
+        sa.Column("created_at", sa.DateTime(timezone=True), nullable=False),
+        sa.Column("last_used_at", sa.DateTime(timezone=True), nullable=True),
+        sa.Column("expires_at", sa.DateTime(timezone=True), nullable=True),
+        sa.Column("revoked_at", sa.DateTime(timezone=True), nullable=True),
+    )
+    op.create_index("ix_api_tokens_user_id", "api_tokens", ["user_id"])
+
+
+def downgrade() -> None:
+    op.drop_index("ix_api_tokens_user_id", "api_tokens")
+    op.drop_table("api_tokens")