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/store/migrations/0002_series.sql

@@ -0,0 +1,45 @@
+-- One row per distinct work, shared by every bookmark that tracks it
+-- (ADR-0003). Keyed (site, series_id), the pair a bookmark key decomposes
+-- into. title/series_url/cover are written once, at creation, and never
+-- again: client-supplied values are ignored once the row exists and the
+-- poller is the only party that may change them. kind and the latest-chapter
+-- fields are last-write-wins like the bookmark's own fields.
+CREATE TABLE series (
+  site               text NOT NULL,
+  series_id          text NOT NULL,
+  title              text NOT NULL DEFAULT '',
+  series_url         text NOT NULL DEFAULT '',
+  cover              text NOT NULL DEFAULT '',
+  kind               text NOT NULL DEFAULT 'manga',
+  latest_chapter     text NOT NULL DEFAULT '',
+  latest_chapter_num double precision,
+  -- When the server last polled this series, unix ms; 0 means never, and sorts
+  -- first so a new bookmark is picked up on the next tick with no special case.
+  latest_checked_at  bigint NOT NULL DEFAULT 0,
+  PRIMARY KEY (site, series_id)
+);
+
+-- Backfill from today's rows. The bookmark key's uniqueness makes
+-- (site, series_id) unique in practice; DISTINCT is belt and braces.
+INSERT INTO series (site, series_id, title, series_url, cover, kind,
+                    latest_chapter, latest_chapter_num, latest_checked_at)
+SELECT DISTINCT site, series_id, title, series_url, cover, kind,
+                latest_chapter, latest_chapter_num, latest_checked_at
+FROM bookmarks;
+
+-- The bookmark keeps only what differs between readers (ADR-0003): progress,
+-- favourite, lifecycle bucket. The dropped columns now live on series.
+ALTER TABLE bookmarks
+  DROP COLUMN title,
+  DROP COLUMN series_url,
+  DROP COLUMN cover,
+  DROP COLUMN kind,
+  DROP COLUMN latest_chapter,
+  DROP COLUMN latest_chapter_num,
+  DROP COLUMN latest_checked_at;
+
+-- A bookmark may not point at a series that does not exist. No cascade: a
+-- series outlives its last bookmark, and deleting one is not a store operation.
+ALTER TABLE bookmarks
+  ADD CONSTRAINT bookmarks_series_fk
+  FOREIGN KEY (site, series_id) REFERENCES series (site, series_id);