feat: ConnectWise PSA integration (#106)

PSA abstraction layer with provider pattern, ConnectWise integration (connection management, ticket linking, note posting, status updates, member mapping), Integrations page UI, Fernet credential encryption, in-memory TTL cache, 6 DB migrations, ConnectWise API reference docs.

frontend/src/api/index.ts

@@ -22,3 +22,4 @@ export { assistantChatApi } from './assistantChat'
 export { flowTransferApi } from './flowTransfer'
 export { kbAcceleratorApi } from './kbAccelerator'
 export { scriptsApi } from './scripts'
+export { integrationsApi, sessionPsaApi } from './integrations'