feat: model registry V2 — provider-aware schema with multi-account support

Adds a providers section to the per-user model registry for Anthropic and
Google as first-class providers alongside local hosts. Google accounts
(API keys) are now stored as a list so multiple Google accounts can coexist.

Changes:
- model_registry.py: V2 schema, auto migration V1→V2 (pulls gemini_api_key
  from auth.json into providers.google.accounts), _resolve_model() merges
  account API key for gemini_api type models
- routers/orchestrator.py: uses model-resolved api_key when orchestrator
  role resolves to a gemini_api model with account_id
- ANTHROPIC_CATALOG and GOOGLE_CATALOG constants for model picker (Phase 2)
- New functions: get_google_api_key(), save/remove_google_account(), get_catalog()
- Documentation: ARCH__BACKENDS.md updated to V2 schema, DESIGN doc added

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

documentation/TODO__Agents.md

@@ -27,6 +27,13 @@ Gemini API orchestrator for private/offline tasks.
 
 ## 🟡 Medium Priority
 
+### [Models] Model Registry V2 — Unified Provider System
+See `DESIGN__Model_Registry_V2.md` for full design.
+- [x] **Phase 1** — V2 schema with providers (Anthropic/Google), multi-account Gemini, auto migration, orchestrator uses account API key — 2026-04-27
+- [ ] **Phase 2** — Cloud provider UI: Anthropic + Google sections in `/settings/models`, account management, model entry creation for cloud models
+- [ ] **Phase 3** — Unified roles + toggle redesign: standalone role assignments, chat toggle cycles role slots (Primary/Backup 1/Backup 2) showing model label
+- [ ] **Phase 4** — Polish: Claude API key, OpenRouter as named provider, catalog sync from API
+
 ### [Intelligence] Knowledge consolidation — Phase 1
 See `ARCH__Intelligence_Layer.md` for full design.
 - [x] Tool: `ae_journal_search` — search before creating to avoid duplicates