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.

REDEPLOY.md

@@ -59,7 +59,7 @@ network can reach it — so every command below goes in through the container:
 
 ```bash
 $COMPOSE exec -T postgres psql -U bookmarks -d bookmarks -c '\dt'
-# -> bookmarks, schema_migrations
+# -> bookmarks, schema_migrations, series
 ```
 
 Inside the container that connects over the local socket as the `bookmarks`
@@ -100,6 +100,7 @@ docker run --rm -v "$BACKUP_DIR":/backup postgres:17-alpine \
   pg_restore --list "/backup/bookmarks-$STAMP.dump" | grep 'TABLE DATA'
 # -> 1234; 0 0 TABLE DATA public bookmarks bookmarks
 # -> 1235; 0 0 TABLE DATA public schema_migrations bookmarks
+# -> 1236; 0 0 TABLE DATA public series series
 
 # 2. Sanity-check the live row count you just captured.
 $COMPOSE exec -T postgres psql -U bookmarks -d bookmarks \