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.go

@@ -315,25 +315,42 @@ func (s *Store) EnsureReader(discordID string, epochZeroHash [32]byte) (int64, e
 	return id, nil
 }
 
+// SightingDisagreementLimit is the number of contradictions that stop that
+// Reader's Sightings from deferring a Poll (issue #103). The counters exist
+// before the mechanism that moves them, so the admin page (issue #102) can
+// clear a false mark without waiting for the Sighting feature.
+const SightingDisagreementLimit = 3
+
+// Blocked reports whether this Reader's Sighting marks have reached the
+// disagreement limit, which stops their Sightings from deferring a Poll.
+func (r ReaderSummary) Blocked() bool {
+	return r.Disagreements >= SightingDisagreementLimit
+}
+
 // ReaderSummary is one Reader as the owner's administration panel sees them:
-// who they are and how many live sessions they hold. No credential material,
-// hashed or otherwise, is exposed.
+// who they are, how many live sessions they hold, and their Sighting marks.
+// No credential material, hashed or otherwise, is exposed.
 type ReaderSummary struct {
 	ID        int64
 	DiscordID string
 	// Sessions counts unexpired session rows — what the owner revokes.
 	Sessions int
+	// Agreements and Disagreements are the Sighting counters (issue #102);
+	// zero means a trusted Reader.
+	Agreements    int
+	Disagreements int
 }
 
-// Readers lists every Reader with their live session count, oldest first, so
-// the owner row (always the oldest) heads the list.
+// Readers lists every Reader with their live session count and Sighting
+// marks, oldest first, so the owner row (always the oldest) heads the list.
 func (s *Store) Readers() ([]ReaderSummary, error) {
 	rows, err := s.db.Query(`
 		SELECT r.id, r.discord_id,
+		       r.sighting_agreements, r.sighting_disagreements,
 		       count(sess.id) FILTER (WHERE sess.expires_at > now()) AS sessions
 		FROM readers r
 		LEFT JOIN sessions sess ON sess.reader_id = r.id
-		GROUP BY r.id, r.discord_id
+		GROUP BY r.id, r.discord_id, r.sighting_agreements, r.sighting_disagreements
 		ORDER BY r.id`)
 	if err != nil {
 		return nil, fmt.Errorf("query readers: %w", err)
@@ -343,7 +360,7 @@ func (s *Store) Readers() ([]ReaderSummary, error) {
 	out := []ReaderSummary{}
 	for rows.Next() {
 		var r ReaderSummary
-		if err := rows.Scan(&r.ID, &r.DiscordID, &r.Sessions); err != nil {
+		if err := rows.Scan(&r.ID, &r.DiscordID, &r.Agreements, &r.Disagreements, &r.Sessions); err != nil {
 			return nil, fmt.Errorf("scan reader: %w", err)
 		}
 		out = append(out, r)
@@ -351,6 +368,18 @@ func (s *Store) Readers() ([]ReaderSummary, error) {
 	return out, rows.Err()
 }
 
+// ClearReaderMarks zeroes a Reader's Sighting counters. It is the owner's
+// remedy for a mark produced by a broken Site adapter rather than a dishonest
+// Reader: it restores a privilege, it is not destruction.
+func (s *Store) ClearReaderMarks(readerID int64) error {
+	if _, err := s.db.Exec(`
+		UPDATE readers SET sighting_agreements = 0, sighting_disagreements = 0
+		WHERE id = $1`, readerID); err != nil {
+		return fmt.Errorf("clear reader marks for reader %d: %w", readerID, err)
+	}
+	return nil
+}
+
 // readersMigration is the version that creates the readers table. The owner
 // seed runs between two migrate passes, so that the run-once migration which
 // attaches existing bookmarks (0004) finds the owner row.