fix(sysadmin): use site_id UUID string instead of integer PK throughout manager endpoints

The sysadmin panel URL uses site.site_id (UUID string) not site.id (int PK).
All manager account endpoints were querying/expecting the integer PK — none matched.

- GET /by-site/{site_id}: param type str, filter by Site.site_id
- POST /register: site_ids type list[str], query Site.site_id.in_()
- DELETE /site-access: site_id type str, query Site.site_id
- schemas/manager.py: ManagerRegisterRequest.site_ids list[int] → list[str]
- SiteDetailPage.jsx: remove Number() casts on siteId in add/remove calls

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

cloud_backend/schemas/manager.py

@@ -8,7 +8,7 @@ class ManagerRegisterRequest(BaseModel):
     email: str
     password: str
     full_name: Optional[str] = None
-    site_ids: list[int] = []
+    site_ids: list[str] = []  # site_id UUID strings, not integer PKs
 
 
 class ManagerLoginRequest(BaseModel):