fix: use BYPASSRLS session for all auth deps and user-mutation endpoints

Phase 4 enabled RLS on the users table. All code paths that touch users
(or other RLS-protected tables) before require_tenant_context sets
app.current_account_id must use get_admin_db (BYPASSRLS):

- deps.py: get_current_user and get_current_active_user → get_admin_db
- auth.py: all endpoints → get_admin_db (login, register, refresh, etc.
  run before tenant context exists; mutation endpoints also need session
  consistency since current_user is in the admin session)
- accounts.py: transfer_ownership, leave_account, delete_account
  → get_admin_db (these mutate current_user directly)
- onboarding.py: dismiss_onboarding → get_admin_db (same reason)

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

backend/app/api/endpoints/accounts.py

@@ -9,6 +9,7 @@ from sqlalchemy import select
 
 from pydantic import BaseModel
 from app.core.database import get_db
+from app.core.admin_database import get_admin_db
 from app.core.subscriptions import get_account_subscription, get_plan_limits, get_account_usage
 from app.core.audit import log_audit
 from app.models.refresh_token import RefreshToken
@@ -148,7 +149,7 @@ async def update_member_role(
 @router.post("/me/transfer-ownership", response_model=AccountResponse)
 async def transfer_ownership(
     data: TransferOwnershipRequest,
-    db: Annotated[AsyncSession, Depends(get_db)],
+    db: Annotated[AsyncSession, Depends(get_admin_db)],
     current_user: Annotated[User, Depends(require_account_owner)]
 ):
     """Transfer account ownership to another member (owner only)."""
@@ -377,7 +378,7 @@ async def list_invites(
 
 @router.post("/me/leave")
 async def leave_account(
-    db: Annotated[AsyncSession, Depends(get_db)],
+    db: Annotated[AsyncSession, Depends(get_admin_db)],
     current_user: Annotated[User, Depends(get_current_active_user)]
 ):
     """Leave the current account (non-owners only). Creates a personal account."""
@@ -423,7 +424,7 @@ class DeleteAccountRequest(BaseModel):
 @router.delete("/me")
 async def delete_account(
     data: DeleteAccountRequest,
-    db: Annotated[AsyncSession, Depends(get_db)],
+    db: Annotated[AsyncSession, Depends(get_admin_db)],
     current_user: Annotated[User, Depends(require_account_owner)]
 ):
     """Delete the current account and soft-delete the user (owner only, no other members)."""