fix(sync): resurrect tombstoned annotations when a newer save re-creates them

Deleting a bookmark/highlight/note and then re-adding the same content
at the same position (same dedup key — e.g. the reader's auto-titled
'Bookmark at X%') was silently swallowed: the save hit the tombstone
branch, returned 201 with the deleted row, and the list (which filters
deleted) stayed empty. Bookmarks were further blocked by the
UNIQUE(media_item_id, user_id, title) slot the tombstoned row holds,
and notes had no TTL escape at all.

Tombstones now only block saves that predate them (stale replays from
a device that still has the annotation). A save whose modification
time is newer than max(deleted_at, last_modified_at) — a deliberate
re-create from the web or a device — resurrects the row via the LWW
update queries, which now clear deleted/deleted_at.

internal/sync/annotations_test.go

@@ -341,3 +341,31 @@ func pgHighlights(text, color, note string, pctStart, pctEnd float64) database.M
 		PercentageEnd:   pgtype.Float8{Float64: pctEnd, Valid: pctEnd != 0},
 	}
 }
+
+func TestIncomingNewerThanTombstone(t *testing.T) {
+	base := time.Date(2026, 8, 14, 12, 0, 0, 0, time.UTC)
+	delAt := pgtype.Timestamptz{Time: base, Valid: true}
+	lastMod := pgtype.Timestamptz{Time: base.Add(-time.Minute), Valid: true}
+
+	tests := []struct {
+		name     string
+		incoming time.Time
+		deleted  pgtype.Timestamptz
+		lastMod  pgtype.Timestamptz
+		want     bool
+	}{
+		{"newer than tombstone resurrects", base.Add(time.Hour), delAt, lastMod, true},
+		{"older than tombstone is a stale replay", base.Add(-time.Hour), delAt, lastMod, false},
+		{"missing timestamp never resurrects", time.Time{}, delAt, lastMod, false},
+		{"exactly equal does not resurrect", base, delAt, lastMod, false},
+		{"last_modified newer than deleted_at wins", base.Add(30 * time.Minute), delAt, pgtype.Timestamptz{Time: base.Add(90 * time.Minute), Valid: true}, false},
+		{"invalid timestamps compare against deleted_at", base.Add(time.Hour), delAt, pgtype.Timestamptz{}, true},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := incomingNewerThanTombstone(tt.incoming, tt.deleted, tt.lastMod); got != tt.want {
+				t.Errorf("incomingNewerThanTombstone() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}