feat(psa): add PsaMemberMapping model and migration

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

backend/app/models/__init__.py

@@ -38,6 +38,7 @@ 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",
@@ -90,4 +91,5 @@ __all__ = [
     "ScriptGeneration",
     "PsaConnection",
     "PsaPostLog",
+    "PsaMemberMapping",
 ]