feat(api): harden CORS and surface 500/409 errors with proper CORS headers

Three independent improvements that fix real browser-facing bugs:

1. CORS: allow 127.0.0.1:5173 in addition to localhost:5173. Both
   resolve to the same Vite dev server but CORS treats them as distinct
   origins, so tabs opened via the IP form silently break.
2. CORS: support CYCLONE_ALLOWED_ORIGINS env var (comma-separated) for
   LAN / staging hosts. The middleware reads it at module import.
3. Catch-all exception handler: returns JSON 500 with CORS headers
   instead of a bare Uvicorn text/plain response. Without this, any
   unhandled exception is misreported by browsers as a CORS error
   because the body can't be read without the allow-origin header.
4. IntegrityError → 409: when (batch_id, patient_control_number) is
   UNIQUE-constrained and a duplicate collides, return 409 with the
   batch id instead of letting the exception 500. Same problem as (3)
   for the most common ingest failure mode.

Tests added:
- test_cors_headers_present_for_loopback_ip
- test_cors_extra_origins_via_env (uses importlib.reload because the
  allow-list is built at module import)

backend/src/cyclone/api.py

@@ -9,8 +9,11 @@ The frontend (Vite/React on http://localhost:5173) uploads an X12 file via
   — one envelope, one line per claim/payout, then one summary — so the
   UI can render records incrementally as they're produced.
 
-CORS is configured to allow the Vite dev origin (``http://localhost:5173``)
-plus GET/POST with any header.
+CORS is configured to allow the Vite dev origins (``http://localhost:5173``
+and ``http://127.0.0.1:5173`` — both forms are valid dev-server addresses
+but CORS treats them as distinct origins) plus GET/POST with any header.
+Additional origins (LAN IPs, staging hosts) can be appended via the
+``CYCLONE_ALLOWED_ORIGINS`` env var as a comma-separated list.
 """
 
 from __future__ import annotations
@@ -19,6 +22,7 @@ import csv
 import io
 import json
 import logging
+import os
 import uuid
 from datetime import datetime, timezone
 from contextlib import asynccontextmanager
@@ -33,6 +37,7 @@ from pydantic import ValidationError
 from cyclone import __version__, db
 from cyclone.db import Claim, ClaimState, Remittance
 from sqlalchemy import desc, or_
+from sqlalchemy.exc import IntegrityError
 from cyclone.inbox_state import apply_999_rejections
 from cyclone.inbox_state_277ca import apply_277ca_rejections
 from cyclone.audit_log import AuditEvent, append_event, verify_chain
@@ -214,7 +219,17 @@ PAYER_FACTORIES_835: dict[str, Any] = {
     "generic_835": PayerConfig835.generic_835,
 }
 
-VITE_DEV_ORIGIN = "http://localhost:5173"
+# Allow both common dev-server origins. ``localhost`` and ``127.0.0.1``
+# resolve to the same Vite dev server but CORS treats them as distinct
+# origins, so the allow-list has to list both — otherwise a tab opened
+# via the IP form gets blocked even though the dev server is identical.
+VITE_DEV_ORIGINS: list[str] = [
+    "http://localhost:5173",
+    "http://127.0.0.1:5173",
+]
+_extra_origins = os.environ.get("CYCLONE_ALLOWED_ORIGINS", "").strip()
+if _extra_origins:
+    VITE_DEV_ORIGINS.extend(o.strip() for o in _extra_origins.split(",") if o.strip())
 
 app = FastAPI(
     title="Cyclone 837P / 835 Parser API",
@@ -225,7 +240,7 @@ app = FastAPI(
 
 app.add_middleware(
     CORSMiddleware,
-    allow_origins=[VITE_DEV_ORIGIN],
+    allow_origins=VITE_DEV_ORIGINS,
     allow_credentials=False,
     allow_methods=["GET", "POST"],
     allow_headers=["*"],
@@ -282,6 +297,35 @@ def _resolve_payer_835(name: str) -> PayerConfig835:
     return PAYER_FACTORIES_835[name]()
 
 
+# --------------------------------------------------------------------------- #
+# Catch-all exception handler
+# --------------------------------------------------------------------------- #
+#
+# Without this, any exception that escapes a route handler is rendered by
+# Uvicorn as a bare ``500 Internal Server Error`` text/plain response with
+# NO CORS headers. Browsers reading such a response report it as a CORS
+# error (``Origin ... is not allowed by Access-Control-Allow-Origin``)
+# because they cannot read the body without the CORS allow-origin header
+# — even though the actual failure was on the server. We catch everything
+# here, log it, and return a JSON error with the request's Origin echoed
+# back so the browser can surface the real message.
+@app.exception_handler(Exception)
+async def _unhandled_exception_handler(request: Request, exc: Exception) -> JSONResponse:
+    log.exception(
+        "Unhandled exception in %s %s", request.method, request.url.path
+    )
+    headers: dict[str, str] = {}
+    origin = request.headers.get("origin", "")
+    if origin:
+        headers["Access-Control-Allow-Origin"] = origin
+        headers["Vary"] = "Origin"
+    return JSONResponse(
+        status_code=500,
+        content={"error": "Internal server error", "detail": str(exc)},
+        headers=headers,
+    )
+
+
 @app.post("/api/parse-837")
 async def parse_837(
     request: Request,
@@ -345,7 +389,30 @@ async def parse_837(
         parsed_at=utcnow(),
         result=result,
     )
-    store.add(rec, event_bus=request.app.state.event_bus)
+    try:
+        store.add(rec, event_bus=request.app.state.event_bus)
+    except IntegrityError as exc:
+        # ``(batch_id, patient_control_number)`` is UNIQUE — fires when a
+        # single batch file contains the same CLM01 control number twice,
+        # or when the same claim id has already been ingested in a prior
+        # batch. Surface as 409 with the batch id so the caller can look
+        # it up; do NOT 500 (a 500 without CORS headers is misreported by
+        # browsers as a CORS error and hides the real cause).
+        log.warning("Duplicate claim while persisting batch %s: %s", rec.id, exc)
+        return JSONResponse(
+            status_code=409,
+            content={
+                "error": "Duplicate claim",
+                "detail": (
+                    "This file (or one previously ingested with the same "
+                    "claim control number) collides with an existing "
+                    "record. Inspect the file for duplicate CLM01 "
+                    "control numbers, or remove the existing batch "
+                    "before retrying."
+                ),
+                "batch_id": rec.id,
+            },
+        )
 
     if _client_wants_json(request):
         body = json.loads(result.model_dump_json())
@@ -516,7 +583,23 @@ async def parse_835_endpoint(
         parsed_at=utcnow(),
         result=result,
     )
-    store.add(rec, event_bus=request.app.state.event_bus)
+    try:
+        store.add(rec, event_bus=request.app.state.event_bus)
+    except IntegrityError as exc:
+        log.warning("Duplicate remittance while persisting batch %s: %s", rec.id, exc)
+        return JSONResponse(
+            status_code=409,
+            content={
+                "error": "Duplicate remittance",
+                "detail": (
+                    "This 835 file (or one previously ingested with the "
+                    "same payer claim control number) collides with an "
+                    "existing record. Remove the existing remittance "
+                    "before retrying."
+                ),
+                "batch_id": rec.id,
+            },
+        )
 
     if _client_wants_json(request):
         body = json.loads(result.model_dump_json())