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.

backend/app/models/__init__.py

@@ -36,6 +36,9 @@ from .survey_response import SurveyResponse
 from .survey_invite import SurveyInvite
 from .kb_import import KBImport, KBImportNode
 from .script_template import ScriptCategory, ScriptTemplate, ScriptGeneration
+from .psa_connection import PsaConnection
+from .psa_post_log import PsaPostLog
+from .psa_member_mapping import PsaMemberMapping
 
 __all__ = [
     "User",
@@ -86,4 +89,7 @@ __all__ = [
     "ScriptCategory",
     "ScriptTemplate",
     "ScriptGeneration",
+    "PsaConnection",
+    "PsaPostLog",
+    "PsaMemberMapping",
 ]