feat(psa): add psa_ticket_id and psa_connection_id to sessions

Add columns to link sessions to PSA tickets and connections. Includes
migration 059, model relationship, and response schema fields.

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

backend/app/schemas/session.py

@@ -94,6 +94,10 @@ class SessionResponse(BaseModel):
     batch_id: Optional[UUID] = None
     target_label: Optional[str] = None
 
+    # PSA ticket link
+    psa_ticket_id: Optional[str] = None
+    psa_connection_id: Optional[UUID] = None
+
     class Config:
         from_attributes = True
 
@@ -140,3 +144,28 @@ class SaveAsTreeResponse(BaseModel):
     tree_id: UUID
     tree_name: str
     message: str
+
+
+# ── PSA ticket link ──────────────────────────────────────────────────
+
+
+class TicketLinkRequest(BaseModel):
+    """Link or unlink a PSA ticket to a session."""
+    psa_ticket_id: Optional[str] = None  # null to unlink
+
+
+class PSATicketResponse(BaseModel):
+    """PSA ticket details returned when linking."""
+    id: str
+    summary: str
+    company_name: Optional[str] = None
+    board_name: Optional[str] = None
+    status_name: Optional[str] = None
+    priority_name: Optional[str] = None
+
+
+class TicketLinkResponse(BaseModel):
+    """Response after linking/unlinking a ticket."""
+    session_id: str
+    psa_ticket_id: Optional[str] = None
+    ticket: Optional[PSATicketResponse] = None