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/migrations/0010_reader_sightings.sql

@@ -0,0 +1,6 @@
+-- The owner's administrative page (issue #102) renders these counters and
+-- offers a control to clear them, deliberately shipped before the Sighting
+-- feature (issue #103) that fills them, so a false mark never needs SQL
+-- against production. Zero counters mean a trusted Reader.
+ALTER TABLE readers ADD COLUMN sighting_agreements integer NOT NULL DEFAULT 0;
+ALTER TABLE readers ADD COLUMN sighting_disagreements integer NOT NULL DEFAULT 0;