store: Sighting counters and the owner's clear control (#102)

The owner's page (issue #102) shows each Reader's Sighting record and offers
one action to wipe it. The counters ship before the Sighting feature that
moves them (issue #103) on purpose: the remedy for a false mark must exist
before marks can be made, or the first broken adapter is fixed with SQL
against production.

- 0010 adds sighting_agreements / sighting_disagreements, both zero-defaulted,
  so every existing Reader reads as trusted.
- ReaderSummary carries both, plus Blocked() against SightingDisagreementLimit,
  so the page states the verdict rather than making the owner derive it.
- ClearReaderMarks zeroes one Reader's pair.

backend/internal/store/store_test.go

@@ -1334,6 +1334,86 @@ func TestReadersAndSessionRevocation(t *testing.T) {
 	}
 }
 
+// The Sighting counters ship before the mechanism that fills them (issue
+// #102 before #103), so the admin page depends on their default: a fresh
+// Reader reads back trusted. Marking one directly proves Readers() reports
+// the counters and Blocked() flips at the limit, and that ClearReaderMarks —
+// the owner's remedy for a false mark — zeroes them again.
+func TestReaderSightingMarks(t *testing.T) {
+	s := newTestStore(t)
+	other := secondReader(t, s)
+
+	readers, err := s.Readers()
+	if err != nil {
+		t.Fatalf("Readers: %v", err)
+	}
+	if len(readers) != 2 {
+		t.Fatalf("readers = %d, want the owner and the second Reader", len(readers))
+	}
+	for _, r := range readers {
+		if r.Agreements != 0 || r.Disagreements != 0 || r.Blocked() {
+			t.Fatalf("fresh reader %d has marks: %+v", r.ID, r)
+		}
+	}
+
+	// The counters' default is the trusted state; writing them directly is
+	// the only way to exercise the read path until issue #103 moves them.
+	if _, err := s.db.Exec(`
+		UPDATE readers SET sighting_agreements = 5, sighting_disagreements = 2
+		WHERE id = $1`, other); err != nil {
+		t.Fatalf("mark reader: %v", err)
+	}
+	readers, err = s.Readers()
+	if err != nil {
+		t.Fatalf("Readers: %v", err)
+	}
+	var marked *ReaderSummary
+	for i := range readers {
+		if readers[i].ID == other {
+			marked = &readers[i]
+		}
+	}
+	if marked == nil || marked.Agreements != 5 || marked.Disagreements != 2 {
+		t.Fatalf("marked reader = %+v, want agreements 5, disagreements 2", marked)
+	}
+	if marked.Blocked() {
+		t.Fatalf("reader with 2 disagreements is blocked; limit is %d", SightingDisagreementLimit)
+	}
+
+	if _, err := s.db.Exec(`
+		UPDATE readers SET sighting_disagreements = 3 WHERE id = $1`, other); err != nil {
+		t.Fatalf("block reader: %v", err)
+	}
+	readers, err = s.Readers()
+	if err != nil {
+		t.Fatalf("Readers: %v", err)
+	}
+	for _, r := range readers {
+		if r.ID == other && !r.Blocked() {
+			t.Fatalf("reader at the disagreement limit is not blocked: %+v", r)
+		}
+		if r.ID == s.OwnerID() && r.Blocked() {
+			t.Fatalf("untouched owner became blocked: %+v", r)
+		}
+	}
+
+	if err := s.ClearReaderMarks(other); err != nil {
+		t.Fatalf("ClearReaderMarks: %v", err)
+	}
+	if err := s.ClearReaderMarks(other + 9999); err != nil {
+		t.Fatalf("ClearReaderMarks(unknown id): %v", err)
+	}
+	readers, err = s.Readers()
+	if err != nil {
+		t.Fatalf("Readers: %v", err)
+	}
+	for _, r := range readers {
+		if r.Agreements != 0 || r.Disagreements != 0 || r.Blocked() {
+			t.Fatalf("reader %d not cleared: %+v", r.ID, r)
+		}
+	}
+}
+
 // Two Readers on one Series: one series row, two independent progresses. The
 // second Reader starts at zero however far the first has read, and the shared
 // row is still due exactly once.