Replace hardcoded `allow_origins=["*"]` with a parsed `CORS_ORIGINS`
environment variable (comma-separated). Default remains `*` for
backward compatibility — no existing deployment breaks — but the API
now logs a startup warning prompting users to set it explicitly for
production.
Exception handlers now route their CORS headers through a shared
`_cors_headers()` helper that mirrors Starlette's CORSMiddleware
behavior: reflects the request Origin when allowed (handling the
browser-rejected `*` + credentials combination correctly), and omits
`Access-Control-Allow-Origin` for disallowed origins so error bodies
don't leak cross-origin when `CORS_ORIGINS` is configured.
Closes#585, #730.
Based on the original work by Greg Grace in #597; rewritten on top of
current main to address prior review feedback (load_dotenv kept at
top, `import os` grouped with stdlib, `_cors_headers` defined before
its exception-handler callers, origins parsed once at module load)
and to choose a non-breaking default paired with a startup warning
instead of a stricter-by-default origin.
Co-authored-by: Greg Grace <ggrace@519lab.com>
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