fix(backend): report ingest correctness — propagate storage failure, 413 on oversize, global body cap (closes #23)

A buffer failure on POST /api/v1/reports now returns 500 instead of a
false `ok`, so clients 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, rejecting an
oversized Content-Length up front and capping the read otherwise; 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. Storage failure uses 500: a full buffer or write error is
server-side and retryable.

Model: opus-5-5

backend/internal/server/export_test.go

@@ -0,0 +1,5 @@
+package server
+
+// MaxRequestBodyBytes exposes the router-wide body limit to the
+// external tests.
+const MaxRequestBodyBytes = maxRequestBodyBytes