Store and compare file mtimes to the nanosecond (closes #226)

The files table held mtime in whole seconds and the scanner compared
whole seconds. A file rewritten with its size unchanged and a new mtime
in the same second as the indexed one was treated as unchanged, and
every later snapshot restored the old content. A new mtime_nsec column
now holds the nanoseconds within the second that mtime holds, and the
scanner compares the full mtime.

A local index created before this change lacks the column and is
rebuilt with `vaultik database delete` and a full backup. A snapshot
made before it cannot be restored by this version.

Model: opus-5-5

internal/database/files_test.go

@@ -304,63 +304,6 @@ func TestFileRepositoryMTimeOutsideInt64NanosecondRange(t *testing.T) {
 	}
 }
 
-// A file already in the index and rewritten within the same second must get
-// its new nanoseconds stored, through both Create and CreateBatch.
-func TestFileRepositoryUpsertMTimeInSameSecond(t *testing.T) {
-	t.Parallel()
-
-	db, cleanup := setupTestDB(t)
-	defer cleanup()
-
-	ctx := context.Background()
-	repo := database.NewFileRepository(db)
-
-	indexed := time.Date(2026, time.October, 7, 12, 0, 0, 100000000, time.UTC)
-	rewritten := indexed.Add(800 * time.Millisecond)
-
-	tests := []struct {
-		name   string
-		upsert func(file *database.File) error
-	}{
-		{"Create", func(file *database.File) error {
-			return repo.Create(ctx, nil, file)
-		}},
-		{"CreateBatch", func(file *database.File) error {
-			return repo.CreateBatch(ctx, nil, []*database.File{file})
-		}},
-	}
-
-	for _, tt := range tests {
-		file := &database.File{
-			ID:    types.NewFileID(),
-			Path:  types.FilePath("/" + tt.name),
-			MTime: indexed,
-		}
-
-		err := tt.upsert(file)
-		if err != nil {
-			t.Fatalf("%s: failed to create file: %v", tt.name, err)
-		}
-
-		file.MTime = rewritten
-
-		err = tt.upsert(file)
-		if err != nil {
-			t.Fatalf("%s: failed to update file: %v", tt.name, err)
-		}
-
-		retrieved, err := repo.GetByPath(ctx, file.Path.String())
-		if err != nil {
-			t.Fatalf("%s: failed to get file: %v", tt.name, err)
-		}
-
-		if !retrieved.MTime.Equal(rewritten) {
-			t.Errorf("%s: mtime got %v, want %v",
-				tt.name, retrieved.MTime, rewritten)
-		}
-	}
-}
-
 func TestFileRepositoryTransaction(t *testing.T) {
 	t.Parallel()