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/alembic/env.py

@@ -20,6 +20,7 @@ from app.models.ai_suggestion import AISuggestion  # noqa: F401
 from app.models.kb_import import KBImport, KBImportNode  # noqa: F401
 from app.models.script_template import ScriptCategory, ScriptTemplate, ScriptGeneration  # noqa: F401
 from app.models.psa_connection import PsaConnection  # noqa: F401
+from app.models.psa_post_log import PsaPostLog  # noqa: F401
 from app.core.config import settings
 
 # this is the Alembic Config object