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/MASTER.md

@@ -1,7 +1,7 @@
 # Cortex / Inara — Master Index
 
 > Start here. This document is a map, not a manual.
-> Last updated: 2026-04-03
+> Last updated: 2026-04-27
 
 ---
 
@@ -24,6 +24,7 @@ Cortex is a self-hosted personal AI platform. It routes messages from any input
 | Gemini backend | ✅ Live | Fallback — via Gemini CLI |
 | Local backend | ✅ Live | Third option — Open WebUI/Ollama on scott_gaming |
 | Gemini orchestrator | ✅ Live | Tool loop → Claude response, Agent mode in UI |
+| Model registry V2 | ✅ Live | Providers (Anthropic/Google/Local), multi-account Gemini |
 | Memory distillation | ✅ Live | Short (daily) / Mid (weekly) / Long (monthly) |
 | Multi-user | ✅ Live | Scott, Holly, Brian — each with own personas |
 | Session search | ✅ Live | Full-text search across past session logs |