feat: Seaca mobile API, enrollment, FCM push and admin UI (0.9.0)

Adds mobile device registration by admin codes, refresh tokens, push channel
in notification policy, and Settings section for managing Seaca clients.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/requirements.txt

@@ -10,6 +10,7 @@ jsonschema>=4.23.0,<5
 passlib[bcrypt]>=1.7.4,<2
 python-jose[cryptography]>=3.3.0,<4
 httpx>=0.28.0,<1
+google-auth>=2.36.0,<3
 psutil>=6.1.0,<8
 
 # dev