Round-2 review findings.
- `_ensure_indexes` swallowed a `PyMongoError` with a bare `pass`. An index
failure does not stop the app serving, but it means the lookup is a
collection scan, which gets slow as the data grows — so it is now logged
as a warning with the traceback.
- The Content-Security-Policy string is now a module-level `CSP` constant
rather than inline in the response handler, and it gained
`object-src 'none'` (the app uses no plugins). The test now also asserts
that neither `unsafe-inline` nor `unsafe-eval` appears — either would
defeat the point of the policy.
- `api_run` returned a bare dict for its 404 while the success path used
`jsonify`. It is now `jsonify({"error": "not found"})` for consistency.