fix(backend): report ingest correctness: 500 on a refused report, 413 on oversize, global body cap (closes #23)

A report the buffer refuses now returns 500 instead of a false `ok`.
Reports reach disk later, so a failed disk write is still answered 200
and shows in the log, and at shutdown as a failed stop with a non-zero
exit. An over-limit body returns 413; malformed JSON stays 400. A
MaxBodyBytes middleware caps every route at 1 MiB; a route group can
only lower that limit. The raw geo blob is no longer logged; client_id,
timestamp and decode error text are cut to 128 bytes before logging.
Panic recovery logs the panic value and stack through slog.

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,