Add LinkedIn OAuth connect with portal-saved app credentials.

Replace manual token paste with Authorization Code flow; store Client ID/secret in the DB from the social accounts UI and surface a copyable callback URL for LinkedIn Auth setup.

.env.example

@@ -68,10 +68,9 @@ PCM_RETURN_ADDRESS=
 # PCM_RETURN_ZIP=
 
 # Social
+# LinkedIn Client ID / Secret are entered in Portal → Social accounts (not env).
 META_APP_ID=
 META_APP_SECRET=
-LINKEDIN_CLIENT_ID=
-LINKEDIN_CLIENT_SECRET=
 # Generate: python -c "from cryptography.fernet import Fernet; print(Fernet.generate_key().decode())"
 SOCIAL_TOKEN_ENCRYPTION_KEY=