feat(psa): add PSA post preview, post-to-ticket, and post history endpoints

Three new session-scoped endpoints for posting session documentation
to linked PSA tickets:

- GET /sessions/{id}/psa-post/preview — generates PSA-formatted content,
  fetches ticket details and available statuses, counts previous posts
- POST /sessions/{id}/psa-post — posts note to CW ticket with optional
  status update, logs all actions in psa_post_log audit trail
- GET /sessions/{id}/psa-posts — returns post history for the session

New schemas: PsaPostRequest, PsaPostResponse, PsaPreviewResponse,
PsaPostLogResponse.

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

backend/app/schemas/__init__.py

@@ -18,6 +18,7 @@ from .script_template import (
 from .psa_connection import (
     PsaConnectionCreate, PsaConnectionUpdate, PsaConnectionResponse, PsaConnectionTestResponse,
     PSATicketSearchResult, PSATicketStatusItem,
+    PsaPostRequest, PsaPostResponse, PsaPreviewResponse, PsaPostLogResponse,
 )
 
 __all__ = [
@@ -46,4 +47,5 @@ __all__ = [
     # PSA Connection
     "PsaConnectionCreate", "PsaConnectionUpdate", "PsaConnectionResponse", "PsaConnectionTestResponse",
     "PSATicketSearchResult", "PSATicketStatusItem",
+    "PsaPostRequest", "PsaPostResponse", "PsaPreviewResponse", "PsaPostLogResponse",
 ]