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>

alembic/versions/0047_series_chapter_dividers.py

@@ -0,0 +1,175 @@
+"""series chapters become cosmetic dividers; pages become one series-global run
+
+FC-6.x reframe (#789). A series is now ONE flat, series-global ordered run of
+pages; chapters stop owning pages and become labeled dividers anchored to the
+page that begins them.
+
+Migration (order matters — series_page.chapter_id cascades, so it must be
+dropped BEFORE any chapter row is deleted, or pages would cascade away):
+  a. Renumber series_page.page_number to a series-global 1..N (ordered by the
+     OLD (chapter_number, page_number)).
+  b. Add series_chapter.anchor_page_id and populate it with each chapter's first
+     page (lowest new page_number).
+  c. Drop series_page.chapter_id (severs the cascade link).
+  d. Prune chapters that shouldn't become dividers: empty/placeholder ones (no
+     anchor) and the redundant unlabeled chapter that would sit at page 1.
+  e. Reshape series_chapter into the divider: drop chapter_number,
+     is_placeholder, stated_page_start/end; make anchor_page_id NOT NULL +
+     UNIQUE + FK→series_page ON DELETE CASCADE.
+
+Revision ID: 0047
+Revises: 0046
+Create Date: 2026-06-11
+
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0047"
+down_revision: Union[str, None] = "0046"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    # a. series-global page numbering, preserving the old reading order.
+    op.execute(
+        """
+        WITH ordered AS (
+            SELECT sp.id,
+                   ROW_NUMBER() OVER (
+                       PARTITION BY sp.series_tag_id
+                       ORDER BY sc.chapter_number, sp.page_number, sp.id
+                   ) AS rn
+            FROM series_page sp
+            JOIN series_chapter sc ON sc.id = sp.chapter_id
+        )
+        UPDATE series_page sp
+        SET page_number = ordered.rn
+        FROM ordered
+        WHERE sp.id = ordered.id
+        """
+    )
+
+    # b. anchor each existing chapter at its first page (lowest new page_number).
+    op.add_column(
+        "series_chapter",
+        sa.Column("anchor_page_id", sa.Integer(), nullable=True),
+    )
+    op.execute(
+        """
+        WITH firsts AS (
+            SELECT DISTINCT ON (sp.chapter_id)
+                   sp.chapter_id, sp.id AS page_id
+            FROM series_page sp
+            ORDER BY sp.chapter_id, sp.page_number, sp.id
+        )
+        UPDATE series_chapter sc
+        SET anchor_page_id = firsts.page_id
+        FROM firsts
+        WHERE firsts.chapter_id = sc.id
+        """
+    )
+
+    # c. sever the ownership link (drops the FK + index with the column) BEFORE
+    #    pruning chapters, so deleting a chapter can't cascade-delete its pages.
+    op.drop_column("series_page", "chapter_id")
+
+    # d. prune chapters that don't become dividers: placeholders / empty ones
+    #    (no anchor), and the unlabeled chapter that would land redundantly at
+    #    page 1 (the series just starts — no divider needed there).
+    op.execute(
+        """
+        DELETE FROM series_chapter sc
+        USING (
+            SELECT sc2.id
+            FROM series_chapter sc2
+            LEFT JOIN series_page sp ON sp.id = sc2.anchor_page_id
+            WHERE sc2.anchor_page_id IS NULL
+               OR (sp.page_number = 1
+                   AND sc2.title IS NULL
+                   AND sc2.stated_part IS NULL)
+        ) gone
+        WHERE sc.id = gone.id
+        """
+    )
+
+    # e. reshape into the divider model.
+    op.drop_column("series_chapter", "chapter_number")
+    op.drop_column("series_chapter", "is_placeholder")
+    op.drop_column("series_chapter", "stated_page_start")
+    op.drop_column("series_chapter", "stated_page_end")
+    op.alter_column("series_chapter", "anchor_page_id", nullable=False)
+    op.create_unique_constraint(
+        "uq_series_chapter_anchor_page", "series_chapter", ["anchor_page_id"]
+    )
+    op.create_foreign_key(
+        "fk_series_chapter_anchor_page",
+        "series_chapter",
+        "series_page",
+        ["anchor_page_id"],
+        ["id"],
+        ondelete="CASCADE",
+    )
+
+
+def downgrade() -> None:
+    # Lossy: dividers can't be reconstructed as owning chapters. Collapse back to
+    # exactly one chapter per series that owns all its pages in order.
+    op.add_column(
+        "series_page", sa.Column("chapter_id", sa.Integer(), nullable=True)
+    )
+    op.drop_constraint(
+        "fk_series_chapter_anchor_page", "series_chapter", type_="foreignkey"
+    )
+    op.drop_constraint(
+        "uq_series_chapter_anchor_page", "series_chapter", type_="unique"
+    )
+    op.drop_column("series_chapter", "anchor_page_id")
+    op.add_column(
+        "series_chapter",
+        sa.Column(
+            "chapter_number", sa.Integer(), nullable=False, server_default="1"
+        ),
+    )
+    op.add_column(
+        "series_chapter",
+        sa.Column(
+            "is_placeholder", sa.Boolean(), nullable=False,
+            server_default="false",
+        ),
+    )
+    op.add_column(
+        "series_chapter",
+        sa.Column("stated_page_start", sa.Integer(), nullable=True),
+    )
+    op.add_column(
+        "series_chapter",
+        sa.Column("stated_page_end", sa.Integer(), nullable=True),
+    )
+    op.execute("DELETE FROM series_chapter")
+    op.execute(
+        """
+        INSERT INTO series_chapter (series_tag_id, chapter_number)
+        SELECT DISTINCT series_tag_id, 1 FROM series_page
+        """
+    )
+    op.execute(
+        """
+        UPDATE series_page sp
+        SET chapter_id = sc.id
+        FROM series_chapter sc
+        WHERE sc.series_tag_id = sp.series_tag_id
+        """
+    )
+    op.alter_column("series_page", "chapter_id", nullable=False)
+    op.create_foreign_key(
+        "fk_series_page_chapter",
+        "series_page",
+        "series_chapter",
+        ["chapter_id"],
+        ["id"],
+        ondelete="CASCADE",
+    )