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>
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