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/dbdump.go

@@ -432,7 +432,12 @@ func ValidateDump(filePath string, dbType DBType) DumpValidation {
 }
 
 // ListDumpFiles returns info about SQL dump files on disk.
-func ListDumpFiles(dumpDir string) ([]DumpFileInfo, error) {
+//
+// M18: ValidateDump scans the dump line-by-line; on a customer with hundreds-of-MB dumps that is wasted
+// disk I/O + CPU on every ~5-min scheduler cycle. `cached` is an optional lookup that returns a
+// previously-computed DumpValidation for a file whose (name, size, modtime) match — when it returns ok,
+// the expensive ValidateDump is skipped. Pass nil to always validate (legacy fast path / other callers).
+func ListDumpFiles(dumpDir string, cached func(name string, size int64, mod time.Time) (DumpValidation, bool)) ([]DumpFileInfo, error) {
 	entries, err := os.ReadDir(dumpDir)
 	if err != nil {
 		if os.IsNotExist(err) {
@@ -474,7 +479,14 @@ func ListDumpFiles(dumpDir string) ([]DumpFileInfo, error) {
 			f.StackName = base
 		}
 
-		// Run validation on the file
+		// M18: reuse a cached validation when the file is unchanged (name+size+modtime), else validate.
+		if cached != nil {
+			if v, ok := cached(e.Name(), info.Size(), info.ModTime()); ok {
+				f.Validation = v
+				files = append(files, f)
+				continue
+			}
+		}
 		fullPath := filepath.Join(dumpDir, e.Name())
 		f.Validation = ValidateDump(fullPath, f.DBType)