Arch: Finalize V3 Auth modularization and Unified Agent spec.

- Integrated zero-dependency Auth models and dependencies_v3.py.
- Successfully resolved circular dependency boot loops.
- Verified site_domain search exception via verify_v3_exceptions.py.
- Refined Unified Agent Architecture with Storage Layer and API-driven access details.
- Updated project roadmap and milestones in GEMINI.md.

app/models/auth_models.py

@@ -0,0 +1,13 @@
+from typing import Optional
+from pydantic import BaseModel
+
+# Zero-dependency auth models for V3
+# Created 2026-01-07 to resolve circular dependencies in FastAPI startup
+
+class AccountContext(BaseModel):
+    account_id: Optional[int]
+    account_id_random: Optional[str]
+    administrator: bool = False
+    manager: bool = False
+    super: bool = False
+    auth_method: str = 'legacy_header'