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",
 ]

backend/app/schemas/psa_connection.py

@@ -64,3 +64,45 @@ class PSATicketStatusItem(BaseModel):
     id: int
     name: str
     is_closed: bool = False
+
+
+# ── PSA post (note posting) schemas ──────────────────────────────
+
+
+class PsaPostRequest(BaseModel):
+    note_type: str = Field(pattern="^(internal_analysis|resolution|description)$")
+    content: str = Field(min_length=1)
+    update_status_id: int | None = None
+
+
+class PsaPostResponse(BaseModel):
+    id: str
+    session_id: str
+    ticket_id: str
+    note_type: str
+    status: str
+    external_note_id: str | None = None
+    error_message: str | None = None
+    status_changed_from: str | None = None
+    status_changed_to: str | None = None
+    posted_at: str
+
+
+class PsaPreviewResponse(BaseModel):
+    content: str
+    ticket: PSATicketSearchResult
+    available_statuses: list[PSATicketStatusItem]
+    character_count: int
+    previous_posts: int
+
+
+class PsaPostLogResponse(BaseModel):
+    id: str
+    ticket_id: str
+    note_type: str
+    status: str
+    error_message: str | None = None
+    status_changed_from: str | None = None
+    status_changed_to: str | None = None
+    posted_at: str
+    content_preview: str  # first 200 chars