feat(backend)!: split Series from Bookmark, keeping the wire format flat

Series becomes a shared row keyed (site, series_id) owning title, cover,
canonical URL, kind, latest chapter and last-checked time (ADR-0003).
A bookmark keeps only progress, favourite, lifecycle bucket, updated_at.

Store.Upsert decomposes one flat body across both tables in one
transaction: client title/series_url/cover apply only when the series row
is new, then only the poll may change them (security boundary — the row
is shared and the values are scraped page content). Reads join series
back in, so GET/PUT emit and accept exactly the flat field set they did
before (ADR-0004), asserted by TestFlatWireFieldSet.

The poller walks Series instead of Bookmarks: one fetch per shared
series per due cycle, due queue ordered reader_count DESC then
latest_checked_at ASC, orphaned series never due and never deleted, row
still stamped before the fetch. Batch/stagger/interval unchanged.

Migration 0002 backfills series from existing bookmarks; verified by
TestMigration0002BackfillsExistingBookmarks.

backend/internal/latest/poller_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"os"
+	"strings"
 	"sync"
 	"testing"
 	"time"
@@ -25,26 +26,35 @@ func newTestStore(t *testing.T) *store.Store {
 	return s
 }
 
-// seedForCheck inserts a bookmark and forces its latest_checked_at.
+// seedForCheck inserts a bookmark (and with it its series) and forces the
+// series' latest_checked_at.
 func seedForCheck(t *testing.T, s *store.Store, key, seriesURL string, checkedAt int64) {
 	t.Helper()
+	site, seriesID, ok := strings.Cut(key, ":")
+	if !ok {
+		t.Fatalf("key %q: no ':' separator", key)
+	}
 	if _, err := s.Upsert(store.Bookmark{
 		Key:       key,
-		Site:      "asura",
-		SeriesID:  key,
+		Site:      site,
+		SeriesID:  seriesID,
 		SeriesURL: seriesURL,
 		UpdatedAt: 1000,
 	}); err != nil {
 		t.Fatalf("seed %q: %v", key, err)
 	}
-	if err := s.MarkLatestChecked(key, checkedAt); err != nil {
+	if err := s.MarkLatestChecked(site, seriesID, checkedAt); err != nil {
 		t.Fatalf("seed mark %q: %v", key, err)
 	}
 }
 
 func readLatestCheckedAt(t *testing.T, s *store.Store, key string) int64 {
 	t.Helper()
-	ts, err := s.LatestCheckedAt(key)
+	site, seriesID, ok := strings.Cut(key, ":")
+	if !ok {
+		t.Fatalf("key %q: no ':' separator", key)
+	}
+	ts, err := s.LatestCheckedAt(site, seriesID)
 	if err != nil {
 		t.Fatalf("LatestCheckedAt %q: %v", key, err)
 	}
@@ -217,6 +227,41 @@ func TestRunOnceRespectsBatchLimit(t *testing.T) {
 	}
 }
 
+// The point of the split (ADR-0003): a series referenced by several bookmarks
+// is fetched once per due cycle, not once per bookmark. Today the bookmark key
+// is <site>:<series_id>, so the second bookmark only exists once keys stop
+// being derived from the series identity (issue #22).
+func TestRunOnceFetchesSharedSeriesOnce(t *testing.T) {
+	s := newTestStore(t)
+	// The slug must match the fixture's own anchors: asura's parser scopes
+	// chapter links to the stored slug.
+	const slug = "chronicles-of-the-demon-faction-f886a8af"
+	const url = "https://asurascans.com/comics/" + slug
+	seedForCheck(t, s, "asura:"+slug, url, 0)
+	if _, err := s.Upsert(store.Bookmark{
+		Key: "asura:" + slug + ":2", Site: "asura", SeriesID: slug, UpdatedAt: 2000,
+	}); err != nil {
+		t.Fatalf("seed second reader: %v", err)
+	}
+
+	f := &fakeFetcher{body: asuraSeriesFixture, status: 200}
+	newTestPoller(t, s, f, time.UnixMilli(5_000_000)).runOnce(context.Background())
+
+	if got := f.callCount(); got != 1 {
+		t.Fatalf("fetched shared series %d times, want 1", got)
+	}
+	// Both bookmarks join to the same updated series row.
+	for _, key := range []string{"asura:" + slug, "asura:" + slug + ":2"} {
+		b, ok, err := s.Get(key)
+		if err != nil || !ok {
+			t.Fatalf("Get %s: %v ok=%v", key, err, ok)
+		}
+		if b.LatestChapterNum == nil || *b.LatestChapterNum != 181 {
+			t.Fatalf("%s LatestChapterNum = %v, want 181", key, b.LatestChapterNum)
+		}
+	}
+}
+
 // One unreachable series must not abandon the rest of the batch.
 func TestRunOnceOneBadSeriesDoesNotStallBatch(t *testing.T) {
 	s := newTestStore(t)
@@ -330,8 +375,8 @@ func TestCheckOneValidatesSeriesURLBeforeFetching(t *testing.T) {
 
 			now := time.UnixMilli(4_000_000)
 			f := &fakeFetcher{body: asuraSeriesFixture, status: 200}
-			newTestPoller(t, s, f, now).checkOne(context.Background(), store.Bookmark{
-				Key: key, Site: tt.site, SeriesURL: tt.seriesURL,
+			newTestPoller(t, s, f, now).checkOne(context.Background(), store.Series{
+				Site: tt.site, SeriesID: "x", SeriesURL: tt.seriesURL,
 			})
 
 			if got := f.callCount(); got != tt.wantCalls {