fix(psa): move CW clientId to server config, remove from user input

ClientId is a product-level GUID registered at developer.connectwise.com,
not per-MSP. Moved to settings.CW_CLIENT_ID env var. MSPs now only
provide site URL, company ID, public key, and private key.

Also added newline handling note to post_note() — CW Developer Guide
states \n is unsupported in JSON bodies. Needs sandbox testing.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/services/psa/registry.py

@@ -8,6 +8,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.models.psa_connection import PsaConnection
 from app.services.psa.base import PSAProvider
+from app.core.config import settings
 from app.services.psa.encryption import decrypt_credentials
 from app.services.psa.exceptions import PSAConnectionError
 
@@ -40,7 +41,7 @@ async def get_provider_for_account(
             company_id=connection.company_id,
             public_key=creds["public_key"],
             private_key=creds["private_key"],
-            client_id=creds["client_id"],
+            client_id=settings.CW_CLIENT_ID or "",
         )
         return ConnectWiseProvider(client)