feat(psa): implement get_ticket in ConnectWise provider

Replace NotImplementedError stub with real implementation that fetches
a ticket by ID via CW REST API and maps it to PSATicket.

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

backend/app/services/psa/connectwise/provider.py

@@ -39,7 +39,24 @@ class ConnectWiseProvider(PSAProvider):
     # ── Stubs for Phase A slices 2-5 ─────────────────────────────────
 
     async def get_ticket(self, ticket_id: str) -> PSATicket:
-        raise NotImplementedError("Implemented in Slice 2")
+        """Fetch a single ticket by ID from ConnectWise."""
+        data = await self.client.get(
+            f"/service/tickets/{ticket_id}",
+            params={"fields": "id,summary,company,board,status,priority,closedFlag"},
+        )
+        return PSATicket(
+            id=str(data["id"]),
+            summary=data.get("summary", ""),
+            company_name=data.get("company", {}).get("name"),
+            company_id=str(data["company"]["id"]) if data.get("company") else None,
+            board_name=data.get("board", {}).get("name"),
+            board_id=data.get("board", {}).get("id"),
+            status_name=data.get("status", {}).get("name"),
+            status_id=data.get("status", {}).get("id"),
+            priority_name=data.get("priority", {}).get("name"),
+            priority_id=data.get("priority", {}).get("id"),
+            closed=data.get("closedFlag", False),
+        )
 
     async def search_tickets(self, query: str, **filters) -> list[PSATicket]:
         raise NotImplementedError("Implemented in Slice 3")