feat(psa): add PsaPostLog model and migration

Audit trail for notes posted to PSA systems. Tracks session ID,
ticket ID, note type, content, status (success/failed), and any
status changes made alongside the note post.

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

backend/app/models/__init__.py

@@ -37,6 +37,7 @@ 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
 
 __all__ = [
     "User",
@@ -88,4 +89,5 @@ __all__ = [
     "ScriptTemplate",
     "ScriptGeneration",
     "PsaConnection",
+    "PsaPostLog",
 ]