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/settings/settings.go

@@ -161,6 +161,11 @@ type DBValidationCache struct {
 	TableCount  int    `json:"table_count"`
 	HasHeader   bool   `json:"has_header"`
 	Error       string `json:"error,omitempty"`
+	// M18: Size + ModTime let ListDumpFiles skip the expensive line-by-line re-validation on every
+	// ~5-min scheduler cycle when the dump file is unchanged. A cache entry is a HIT only when both the
+	// file size and (RFC3339, second-precision) modtime match the on-disk file.
+	Size    int64  `json:"size,omitempty"`
+	ModTime string `json:"mod_time,omitempty"` // RFC3339 (UTC)
 }
 
 // SetDebug enables or disables debug logging for settings operations.