Multi-user auth via gnexus-auth OAuth + hybrid role/permission model ...

- Integrate gnexus-auth-client-py (GAuthClient) for OAuth flow, token refresh,
  and webhook parsing
- Add navi/auth/ package: User model, Fernet encryptor, client singleton,
  deps (get_current_user, require_admin, require_permission)
- New tables: navi_users, user_auth_sessions (auto-created on startup)
- Session/memory isolation by user_id with legacy NULL support
- Cookie-based auth proxy: /auth/login, /callback, /logout, /me
- Webhook receiver /webhooks/gnexus-auth handling user events, global logout,
  session revocation, role/permission changes
- Admin endpoints (/admin/*) gated by role + permissions
- Webclient auth store with isAdmin/hasPermission guards
- Admin-only profile filtering in /agents/profiles
- 200/200 tests passing

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Split memory/store.py into focused mixins ...

- _ddl.py: table creation (conditional on pgvector/pg_trgm)
- _embeddings.py: EmbeddingMixin — pgvector checks, embed generation, backfill
- _facts.py: FactMixin — upsert, search (vector + ILIKE), delete, list, count
- _summary.py: SummaryMixin — get/set conversation summary
- _session_state.py: SessionStateMixin — extraction tracking per session
- store.py: reduced to ~60-line facade composing all mixins

No external API changes — MemoryStore remains the single interface.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>