Adds an `is_admin` flag on users plus the substrate every later PR in the
admin feature builds on:
- migration 0018 adds the column with default false
- `repo::user::bootstrap_admin` creates or promotes the user named by
`ADMIN_USERNAME` at startup, hashing `ADMIN_PASSWORD` only when the row
is new — never overwriting an existing hash, so an operator can rotate
the admin password via the UI without env-var conflict
- `CurrentSessionUser` extractor accepts only the session cookie;
`RequireAdmin` composes over it and additionally requires
`user.is_admin`. Bearer tokens are intentionally excluded so an
admin's bot token never inherits admin authority (privilege-escalation
surface that bites every "API keys reuse user perms" auth design)
- demotion is instant: `RequireAdmin` re-reads the user row each request
`/api/v1/auth/me` now exposes `is_admin`; no other response embeds
`User`, so no privacy fanout to audit.
