M18: cache DB-dump validation; skip re-validate on unchanged dumps

ListDumpFiles ran ValidateDump (line-by-line scan) for every dump on every ~5-min
RefreshCache cycle — wasted I/O+CPU on large customer dumps. ListDumpFiles now takes
an optional cached(name,size,mod) lookup; on a (size+modtime) match it reuses the
prior result and skips ValidateDump. settings.DBValidationCache gains Size+ModTime;
listAllDumpFiles builds the lookup from the persisted cache and writes back only fresh
validations (cache miss), so an unchanged dump triggers neither a re-validation nor a
settings.json write each cycle. nil cached = legacy validate-always (back-compat).
Tests: cache-hit skips validate (sentinel), cache-miss validates, nil validates.

controller/internal/appbackup/listdumpfiles_cache_test.go

@@ -0,0 +1,94 @@
+package appbackup
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+)
+
+// writeDumpFile creates a minimal VALID postgres dump so a real ValidateDump would set Valid=true and
+// TableCount=1 — distinguishable from the sentinel the cache returns.
+func writeDumpFile(t *testing.T, dir string) string {
+	t.Helper()
+	path := filepath.Join(dir, "romm-postgres.sql")
+	body := "-- PostgreSQL database dump\n" +
+		"CREATE TABLE public.t (id int);\n" +
+		"-- PostgreSQL database dump complete\n" +
+		// pad past the 100-byte floor
+		"-- padding ------------------------------------------------------------\n"
+	if err := os.WriteFile(path, []byte(body), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	return path
+}
+
+// TestListDumpFiles_CacheHitSkipsValidate asserts M18: when the `cached` lookup returns ok for an
+// unchanged file, ListDumpFiles reuses that result and does NOT re-run the (expensive) ValidateDump.
+// Proven via a sentinel TableCount=999 that only the cache could supply (a real validate yields 1).
+func TestListDumpFiles_CacheHitSkipsValidate(t *testing.T) {
+	dir := t.TempDir()
+	writeDumpFile(t, dir)
+
+	const sentinel = 999
+	calls := 0
+	cached := func(name string, size int64, mod time.Time) (DumpValidation, bool) {
+		calls++
+		return DumpValidation{Valid: true, TableCount: sentinel}, true // always "hit"
+	}
+
+	files, err := ListDumpFiles(dir, cached)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(files) != 1 {
+		t.Fatalf("expected 1 dump file, got %d", len(files))
+	}
+	if files[0].Validation.TableCount != sentinel {
+		t.Fatalf("validation TableCount = %d, want %d (sentinel from cache) — ValidateDump was re-run instead of using the cache",
+			files[0].Validation.TableCount, sentinel)
+	}
+	if calls != 1 {
+		t.Fatalf("cached lookup called %d times, want 1", calls)
+	}
+}
+
+// TestListDumpFiles_CacheMissValidates asserts that on a cache MISS (e.g. modtime changed) ListDumpFiles
+// falls through to a real ValidateDump (sentinel must NOT appear; real TableCount=1).
+func TestListDumpFiles_CacheMissValidates(t *testing.T) {
+	dir := t.TempDir()
+	writeDumpFile(t, dir)
+
+	cached := func(name string, size int64, mod time.Time) (DumpValidation, bool) {
+		return DumpValidation{Valid: true, TableCount: 999}, false // always "miss"
+	}
+
+	files, err := ListDumpFiles(dir, cached)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(files) != 1 {
+		t.Fatalf("expected 1 dump file, got %d", len(files))
+	}
+	if files[0].Validation.TableCount == 999 {
+		t.Fatalf("got the sentinel on a cache MISS — should have run a real ValidateDump")
+	}
+	if !files[0].Validation.Valid || files[0].Validation.TableCount != 1 {
+		t.Fatalf("real validation expected Valid=true TableCount=1, got Valid=%v TableCount=%d",
+			files[0].Validation.Valid, files[0].Validation.TableCount)
+	}
+}
+
+// TestListDumpFiles_NilCachedAlwaysValidates asserts the legacy fast path (cached==nil) still validates.
+func TestListDumpFiles_NilCachedAlwaysValidates(t *testing.T) {
+	dir := t.TempDir()
+	writeDumpFile(t, dir)
+
+	files, err := ListDumpFiles(dir, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(files) != 1 || !files[0].Validation.Valid || files[0].Validation.TableCount != 1 {
+		t.Fatalf("nil-cached path should validate: got %+v", files[0].Validation)
+	}
+}