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`: 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

backend/internal/middleware/export_test.go

@@ -1,6 +1,7 @@
 package middleware
 
 import (
+	"log/slog"
 	"net/http"
 	"net/netip"
 )
@@ -8,6 +9,12 @@ import (
 // Test-only wrappers exposing unexported helpers to the
 // external middleware_test package.
 
+// NewWithLogger builds a Middleware around a logger for tests
+// that exercise the logging paths without the fx graph.
+func NewWithLogger(log *slog.Logger) *Middleware {
+	return &Middleware{log: log}
+}
+
 func ClientIP(
 	remoteAddr string,
 	header http.Header,