A buffer failure on POST /api/v1/reports now returns 500 instead of a
false `ok`: the failure is server-side and a client can retry. An
over-limit body returns 413 (errors.As on `*http.MaxBytesError`);
malformed JSON stays 400. A MaxBodyBytes middleware (1 MiB) caps every
route; a route group can only lower that limit. The raw geo blob is no
longer logged, only its length; client_id, timestamp and decode error
text are length-bounded before logging. A decodeJSON handler helper is
added. Panic recovery routes the stack through slog as structured
JSON. Writing a report file now returns its error, so a failed final
flush fails the stop and the process exits non-zero.
Model: opus-5-5
