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/handlers/export_test.go

@@ -0,0 +1,13 @@
+package handlers
+
+import "log/slog"
+
+// MaxLoggedFieldBytes exposes the log bound to the external tests.
+const MaxLoggedFieldBytes = maxLoggedFieldBytes
+
+// NewForTest builds a Handlers around a report sink and logger,
+// bypassing the fx graph so handler behaviour (including the
+// storage failure path) is exercisable in unit tests.
+func NewForTest(buf reportAppender, log *slog.Logger) *Handlers {
+	return &Handlers{buf: buf, log: log}
+}