feat(api): GET /api/claims/{claim_id} returns full claim context

backend/src/cyclone/api.py

@@ -712,6 +712,35 @@ def list_claims(
     }
 
 
+@app.get("/api/claims/{claim_id}")
+def get_claim_detail_endpoint(claim_id: str) -> dict:
+    """Return one claim with full drawer context.
+
+    SP4 detail-drawer endpoint. The body is the spec-shaped dict
+    produced by :meth:`CycloneStore.get_claim_detail` — header, state,
+    service lines, diagnoses, parties, validation, raw segments,
+    ``stateHistory`` (most-recent-first, capped at 50), and a
+    populated ``matchedRemittance`` block if the claim is paired.
+
+    Path param is ``claim_id`` (not ``id``) to avoid shadowing
+    FastAPI's internal ``id`` name and to keep OpenAPI docs self-
+    describing (matches the ``/api/acks/{ack_id}`` convention from
+    SP3). Returns 404 when the claim is missing — never 500 — so the
+    UI can distinguish "claim doesn't exist" from a transient fetch
+    failure.
+    """
+    body = store.get_claim_detail(claim_id)
+    if body is None:
+        raise HTTPException(
+            status_code=404,
+            detail={
+                "error": "Not found",
+                "detail": f"Claim {claim_id} not found",
+            },
+        )
+    return body
+
+
 @app.get("/api/reconciliation/unmatched")
 def get_reconciliation_unmatched() -> dict:
     """Return unmatched Claims (left) and unmatched Remittances (right).