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/schemas/psa_connection.py

@@ -12,7 +12,7 @@ class PsaConnectionCreate(BaseModel):
     company_id: str = Field(min_length=1, max_length=100)
     public_key: str = Field(min_length=1)
     private_key: str = Field(min_length=1)
-    client_id: str = Field(min_length=1)
+    # Note: client_id is NOT per-MSP — it's a product-level GUID from settings.CW_CLIENT_ID
 
 
 class PsaConnectionUpdate(BaseModel):
@@ -21,7 +21,6 @@ class PsaConnectionUpdate(BaseModel):
     company_id: str | None = None
     public_key: str | None = None
     private_key: str | None = None
-    client_id: str | None = None
 
 
 class PsaConnectionResponse(BaseModel):