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/reportbuf/reportbuf_test.go

@@ -1,6 +1,8 @@
 package reportbuf_test
 
 import (
+	"errors"
+	"io/fs"
 	"os"
 	"strings"
 	"testing"
@@ -52,6 +54,46 @@ func TestFlushOnShutdown(t *testing.T) {
 	}
 }
 
+// TestFailedFinalFlushFailsStop proves a final flush that cannot
+// write its file makes the stop fail, which makes the process
+// exit non-zero instead of dropping the buffered reports silently.
+func TestFailedFinalFlushFailsStop(t *testing.T) {
+	dir := t.TempDir()
+	t.Setenv("DATA_DIR", dir)
+
+	var buf *reportbuf.Buffer
+
+	app := fxtest.New(t,
+		fx.Provide(
+			globals.New,
+			logger.New,
+			config.New,
+			reportbuf.New,
+		),
+		fx.Populate(&buf),
+	)
+
+	app.RequireStart()
+
+	err := buf.Append(map[string]string{"probe": "shutdown"})
+	if err != nil {
+		t.Fatalf("append report: %v", err)
+	}
+
+	// Removing the data directory leaves the final flush nowhere to
+	// write. A read-only directory would not do: tests run as root
+	// in the backend image, and root ignores the read-only bit.
+	err = os.RemoveAll(dir)
+	if err != nil {
+		t.Fatalf("remove data dir: %v", err)
+	}
+
+	err = app.Stop(t.Context())
+	if !errors.Is(err, fs.ErrNotExist) {
+		t.Fatalf("stop error = %v, want the final flush's error", err)
+	}
+}
+
 func hasReportFile(t *testing.T, dir string) bool {
 	t.Helper()