feat(sp8): GET /api/claims/{id}/serialize-837 endpoint

Returns the persisted ClaimOutput as a regenerated X12 837P file via
the new outbound serializer (Approach A — full rebuild from canonical
fields). 404 on missing claim, 422 if the stored raw_json cannot be
validated as a ClaimOutput. text/x12 content-type, attachment
disposition with the claim id as the filename.

3 tests:
- endpoint returns text/x12 attachment starting with ISA*
- 404 for missing claim id
- regenerated text round-trips back through parse()

backend/src/cyclone/api.py

@@ -28,7 +28,7 @@ from typing import Any, AsyncIterator
 
 from fastapi import FastAPI, File, HTTPException, Query, Request, UploadFile
 from fastapi.middleware.cors import CORSMiddleware
-from fastapi.responses import JSONResponse, StreamingResponse
+from fastapi.responses import JSONResponse, Response, StreamingResponse
 from pydantic import ValidationError
 
 from cyclone import __version__, db
@@ -54,6 +54,7 @@ from cyclone.parsers.parse_999 import parse_999_text
 from cyclone.parsers.parse_ta1 import parse_ta1_text
 from cyclone.parsers.serialize_270 import serialize_270
 from cyclone.parsers.serialize_999 import serialize_999
+from cyclone.parsers.serialize_837 import SerializeError as SerializeError837, serialize_837
 from cyclone.parsers.batch_ack_builder import build_ack_for_batch
 from cyclone.parsers.validator_835 import validate as validate_835
 from cyclone.pubsub import EventBus
@@ -1212,6 +1213,58 @@ def get_claim_detail_endpoint(claim_id: str) -> dict:
     return body
 
 
+@app.get("/api/claims/{claim_id}/serialize-837")
+def serialize_claim_as_837(claim_id: str):
+    """Return the claim as a regenerated X12 837P file (SP8).
+
+    Loads the ClaimOutput from the persisted ``raw_json`` and runs the
+    outbound serializer. Returns 404 if the claim doesn't exist, 422 if
+    the stored payload has no parseable ClaimOutput (data integrity
+    issue, not a transient failure).
+    """
+    with db.SessionLocal()() as s:
+        row = s.get(Claim, claim_id)
+        if row is None:
+            return JSONResponse(
+                {"error": "Not found", "detail": f"Claim {claim_id} not found"},
+                status_code=404,
+            )
+        if not row.raw_json:
+            return JSONResponse(
+                {
+                    "error": "Unprocessable",
+                    "detail": f"Claim {claim_id} has no raw_json; cannot serialize",
+                },
+                status_code=422,
+            )
+        try:
+            claim_obj = ClaimOutput.model_validate(row.raw_json)
+        except Exception as exc:
+            return JSONResponse(
+                {
+                    "error": "Unprocessable",
+                    "detail": f"Claim {claim_id} raw_json is malformed: {exc}",
+                },
+                status_code=422,
+            )
+
+    try:
+        text = serialize_837(claim_obj)
+    except SerializeError837 as exc:
+        return JSONResponse(
+            {"error": "Unprocessable", "detail": str(exc)},
+            status_code=422,
+        )
+
+    return Response(
+        content=text,
+        media_type="text/x12",
+        headers={
+            "Content-Disposition": f'attachment; filename="claim-{claim_id}.x12"'
+        },
+    )
+
+
 @app.get("/api/claims/{claim_id}/line-reconciliation")
 def get_claim_line_reconciliation(claim_id: str) -> dict:
     """Per-line reconciliation view for the ClaimDrawer tab.