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/backup/appbackup_bridge.go

@@ -14,6 +14,7 @@ package backup
 import (
 	"context"
 	"log"
+	"time"
 
 	"gitea.dooplex.hu/admin/felhom-controller/internal/appbackup"
 )
@@ -68,8 +69,8 @@ func ValidateDump(filePath string, dbType DBType) DumpValidation {
 	return appbackup.ValidateDump(filePath, dbType)
 }
 
-func ListDumpFiles(dumpDir string) ([]DumpFileInfo, error) {
-	return appbackup.ListDumpFiles(dumpDir)
+func ListDumpFiles(dumpDir string, cached func(name string, size int64, mod time.Time) (DumpValidation, bool)) ([]DumpFileInfo, error) {
+	return appbackup.ListDumpFiles(dumpDir, cached)
 }
 
 // --- function forwarders (appdata) ---