feat: a Reader's Sighting defers a Poll of a solitary Series (#103)

A userscript PUT already carries the Latest Chapter the Reader's own browser
read; it now stands in for a Poll where being wrong can hurt nobody else.
Deferral lives in the due query beside the rest cutoff: one Bookmark, sighted
within one rest, under the six-rest ceiling. checkOne judges the Reader whose
report raised the value off the comparison it already makes - three
contradictions stop them deferring, twenty confirmations forgive.

ADR-0011 records the trust model and the rejected alternatives.

backend/internal/store/migrations/0011_series_sightings.sql

@@ -0,0 +1,10 @@
+-- Sighting deferral (issue #103). latest_sighted_at is when a Reader's report
+-- last stood in for a Poll; it is separate from latest_checked_at because the
+-- six-hour ceiling has to know when the Series was last really fetched, and a
+-- Sighting writing the Poll's own column would erase that.
+-- latest_raised_by is attribution: whoever last raised this Series' Latest
+-- Chapter by Sighting, so a Poll that contradicts the value downwards names a
+-- Reader rather than flagging a row. Cleared by the Poll that judges it, NULL
+-- whenever the stored value is the Poll's own.
+ALTER TABLE series ADD COLUMN latest_sighted_at bigint NOT NULL DEFAULT 0;
+ALTER TABLE series ADD COLUMN latest_raised_by bigint REFERENCES readers(id) ON DELETE SET NULL;