drop users.role column: full RBAC through group membership

The role column was a pre-RBAC artifact. All authorization now flows
through explicit group membership and permission grants:

- Everyone group: all users added on creation (no implicit membership)
- Admins group: grants surface.admin.access + all platform permissions
- JWT claims, login response, profile: role field removed
- OIDC: isIdPAdmin() maps IdP claims → Admins group (no role writes)
- Admin UI: role dropdown removed, admin managed through groups
- Middleware cache simplified to isActive only

28 files changed, -79 lines net. Zero magic roles.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/database/migrations/sqlite/001_core.sql

@@ -12,7 +12,6 @@ CREATE TABLE IF NOT EXISTS users (
     password_hash   TEXT,
     display_name    TEXT,
     avatar_url      TEXT,
-    role            TEXT DEFAULT 'user' CHECK (role IN ('user', 'admin')),
     is_active       INTEGER DEFAULT 1,
     settings        TEXT DEFAULT '{}',
     auth_source     TEXT NOT NULL DEFAULT 'builtin',