feat(series): flat series sequence + cosmetic chapter dividers (#789 Phase 1)

Reframe a series from "ordered chapters that own pages" to ONE flat,
series-global ordered run of pages with optional cosmetic chapter DIVIDERS
over it. A chapter no longer wraps content — it's a labeled divider anchored
to the page that begins it; a page's chapter is derived as the nearest
preceding divider. This is what lets installments assembled from multiple
sources sit in one continuous, correctly-numbered sequence (operator's
Goblin Juice case).

- migration 0047: flatten each series to a series-global page_number
  (preserving today's reading order); convert each existing chapter to a
  divider anchored at its first page (keeping title/stated_part); drop
  series_page.chapter_id; reshape series_chapter (anchor_page_id UNIQUE FK,
  drop chapter_number/is_placeholder/stated_page_start/end). Loss-safe for
  content; drops empty placeholder chapters + a redundant page-1 divider.
- series_page: page_number is now the series-global order; no chapter_id.
- series_chapter: anchored divider (anchor_page_id, title, stated_part).
- series_service: flat list_pages (one run + derived dividers + per-page
  source_post + part_gaps), series-wide reorder/renumber, divider CRUD
  (create/update/move/delete); retired per-chapter reorder/merge/placement.
- api/tags: drop chapter_id from add; /chapters endpoints are divider
  create/update/delete (removed chapter reorder/merge/page-reorder).
- series_match_service: series "end" reads max(series_page.stated_page);
  accept appends via add_post. tag_service series-merge appends src's pages
  after tgt's max so the merged series stays one clean run.
- frontend: seriesManage store + SeriesManageView → one continuous
  drag-reorder grid with inline divider bars + series-global page numbers;
  reader walks the flat run, headings from dividers; PostSeriesMenu copy.
- tests reworked across the series suite for the divider model.

Phase 2 (pending staging for add-from-post) is separate.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/api/tags.py

@@ -391,14 +391,9 @@ async def series_add(tag_id: int):
     ids, err = _parse_bulk_ids(body, max_ids=500)
     if err:
         return err
-    chapter_id, cerr = _opt_int(body, "chapter_id")
-    if cerr:
-        return cerr
     async with get_session() as session:
         try:
-            n = await SeriesService(session).add_images(
-                tag_id, ids, chapter_id=chapter_id
-            )
+            n = await SeriesService(session).add_images(tag_id, ids)
         except SeriesError as exc:
             return _series_err(exc)
         await session.commit()
@@ -453,30 +448,30 @@ async def series_cover(tag_id: int):
     return jsonify({"ok": True})
 
 
-# ---- chapters (FC-6.1) ----------------------------------------------------
+# ---- chapter dividers (FC-6.x) -------------------------------------------
+# A chapter is a cosmetic divider anchored to the page that begins it; it owns
+# no pages. Page ordering is series-wide (the /reorder endpoint above), so there
+# is no per-chapter reorder/merge/chapter-reorder — those are gone.
 
 
 @tags_bp.route("/series/<int:tag_id>/chapters", methods=["POST"])
 async def series_chapter_create(tag_id: int):
     body = await request.get_json() or {}
+    anchor, aerr = _opt_int(body, "anchor_image_id")
+    if aerr:
+        return aerr
+    if anchor is None:
+        return jsonify({"error": "anchor_image_id required"}), 400
     title = body.get("title")
     if title is not None and not isinstance(title, str):
         return jsonify({"error": "title must be a string"}), 400
-    is_placeholder = bool(body.get("is_placeholder", False))
-    start, serr = _opt_int(body, "stated_page_start")
-    if serr:
-        return serr
-    end, eerr = _opt_int(body, "stated_page_end")
-    if eerr:
-        return eerr
+    part, perr = _opt_int(body, "stated_part")
+    if perr:
+        return perr
     async with get_session() as session:
         try:
-            ch = await SeriesService(session).create_chapter(
-                tag_id,
-                title=title,
-                is_placeholder=is_placeholder,
-                stated_page_start=start,
-                stated_page_end=end,
+            ch = await SeriesService(session).create_divider(
+                tag_id, anchor, title=title, stated_part=part,
             )
         except SeriesError as exc:
             return _series_err(exc)
@@ -484,21 +479,6 @@ async def series_chapter_create(tag_id: int):
     return jsonify(ch)
 
 
-@tags_bp.route("/series/<int:tag_id>/chapters/reorder", methods=["POST"])
-async def series_chapter_reorder(tag_id: int):
-    body = await request.get_json()
-    ids, err = _parse_int_list(body, "chapter_ids")
-    if err:
-        return err
-    async with get_session() as session:
-        try:
-            await SeriesService(session).reorder_chapters(tag_id, ids)
-        except SeriesError as exc:
-            return _series_err(exc)
-        await session.commit()
-    return jsonify({"ok": True})
-
-
 @tags_bp.route(
     "/series/<int:tag_id>/chapters/<int:chapter_id>", methods=["PATCH"]
 )
@@ -514,19 +494,18 @@ async def series_chapter_update(tag_id: int, chapter_id: int):
         if perr:
             return perr
         kwargs.update(set_part=True, stated_part=part)
-    if "stated_page_start" in body:
-        start, serr = _opt_int(body, "stated_page_start")
-        if serr:
-            return serr
-        kwargs.update(set_start=True, stated_page_start=start)
-    if "stated_page_end" in body:
-        end, eerr = _opt_int(body, "stated_page_end")
-        if eerr:
-            return eerr
-        kwargs.update(set_end=True, stated_page_end=end)
+    if "anchor_image_id" in body:
+        anchor, aerr = _opt_int(body, "anchor_image_id")
+        if aerr:
+            return aerr
+        if anchor is None:
+            return jsonify(
+                {"error": "anchor_image_id must be an integer"}
+            ), 400
+        kwargs.update(set_anchor=True, anchor_image_id=anchor)
     async with get_session() as session:
         try:
-            await SeriesService(session).update_chapter(
+            await SeriesService(session).update_divider(
                 tag_id, chapter_id, **kwargs
             )
         except SeriesError as exc:
@@ -541,45 +520,7 @@ async def series_chapter_update(tag_id: int, chapter_id: int):
 async def series_chapter_delete(tag_id: int, chapter_id: int):
     async with get_session() as session:
         try:
-            await SeriesService(session).delete_chapter(tag_id, chapter_id)
-        except SeriesError as exc:
-            return _series_err(exc)
-        await session.commit()
-    return jsonify({"ok": True})
-
-
-@tags_bp.route(
-    "/series/<int:tag_id>/chapters/<int:chapter_id>/merge", methods=["POST"]
-)
-async def series_chapter_merge(tag_id: int, chapter_id: int):
-    body = await request.get_json()
-    target, terr = _opt_int(body, "target_chapter_id")
-    if terr:
-        return terr
-    if target is None:
-        return jsonify({"error": "target_chapter_id required"}), 400
-    async with get_session() as session:
-        try:
-            moved = await SeriesService(session).merge_chapter(
-                tag_id, chapter_id, target
-            )
-        except SeriesError as exc:
-            return _series_err(exc)
-        await session.commit()
-    return jsonify({"moved_count": moved})
-
-
-@tags_bp.route(
-    "/series/<int:tag_id>/chapters/<int:chapter_id>/reorder", methods=["POST"]
-)
-async def series_chapter_reorder_pages(tag_id: int, chapter_id: int):
-    body = await request.get_json()
-    ids, err = _parse_bulk_ids(body, max_ids=500)
-    if err:
-        return err
-    async with get_session() as session:
-        try:
-            await SeriesService(session).reorder_pages(tag_id, chapter_id, ids)
+            await SeriesService(session).delete_divider(tag_id, chapter_id)
         except SeriesError as exc:
             return _series_err(exc)
         await session.commit()
@@ -635,7 +576,7 @@ async def series_add_post(tag_id: int):
         return jsonify({"error": "post_id required"}), 400
     async with get_session() as session:
         try:
-            out = await SeriesService(session).add_post_as_chapter(tag_id, post_id)
+            out = await SeriesService(session).add_post(tag_id, post_id)
         except SeriesError as exc:
             return _series_err(exc)
         await session.commit()