feat(series): chapter layer over series_page — backend (FC-6.1)

Adds an ordered chapter layer to series. Reading order becomes
(series_chapter.chapter_number, series_page.page_number); a chapter may be a
placeholder reserving a slot, and carries an optional parsed stated-page range
used to flag missing-page gaps. An image still lives in at most one series ⇒ one
chapter (image_id stays UNIQUE).

- models: series_chapter; series_page gains chapter_id (NOT NULL, cascade) +
  stated_page. Migration 0040 backfills every existing series into one
  auto-chapter holding its current flat pages — no data loss.
- SeriesService: chapter CRUD (create/update/reorder/delete/merge), page→chapter
  assignment, reorder_pages, chapter-aware set_cover; list_pages now returns
  chapters[] + gaps[] alongside a back-compat flat pages[]. Legacy series-wide
  reorder operates on the single default chapter and rejects multi-chapter series.
- API: chapter endpoints under /api/series/<tag>/chapters; POST pages accepts an
  optional chapter_id.
- TagService.merge now repoints series_chapter too, so a merged series' chapters
  (and their pages) survive the source tag's deletion instead of cascading away.
- Tests: new chapter suite; updated the 4 direct SeriesPage(...) constructions to
  supply chapter_id.

Frontend (chapter-aware manage view + reader) lands next; until then the
existing UI keeps working via the flat pages[] + single default chapter.

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

alembic/versions/0040_series_chapters.py

@@ -0,0 +1,108 @@
+"""series chapters: chapter layer over series_page (FC-6.1)
+
+Revision ID: 0040
+Revises: 0039
+Create Date: 2026-06-07
+
+A series (Tag kind='series') gains an ordered chapter layer. Reading order
+becomes (series_chapter.chapter_number, series_page.page_number). Every existing
+series is backfilled into a single auto-chapter (chapter_number=1) holding its
+current flat pages, so no data is lost and the old flat ordering is preserved.
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0040"
+down_revision: Union[str, None] = "0039"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "series_chapter",
+        sa.Column("id", sa.Integer, primary_key=True),
+        sa.Column(
+            "series_tag_id",
+            sa.Integer,
+            sa.ForeignKey("tag.id", ondelete="CASCADE"),
+            nullable=False,
+        ),
+        sa.Column("chapter_number", sa.Integer, nullable=False),
+        sa.Column("title", sa.Text, nullable=True),
+        sa.Column(
+            "is_placeholder", sa.Boolean, nullable=False, server_default="false"
+        ),
+        sa.Column("stated_page_start", sa.Integer, nullable=True),
+        sa.Column("stated_page_end", sa.Integer, nullable=True),
+        sa.Column(
+            "created_at",
+            sa.DateTime(timezone=True),
+            nullable=False,
+            server_default=sa.text("now()"),
+        ),
+        sa.Column(
+            "updated_at",
+            sa.DateTime(timezone=True),
+            nullable=False,
+            server_default=sa.text("now()"),
+        ),
+    )
+    op.create_index(
+        "ix_series_chapter_series_tag_id", "series_chapter", ["series_tag_id"]
+    )
+
+    # New columns on series_page; chapter_id starts nullable so we can backfill.
+    op.add_column(
+        "series_page", sa.Column("chapter_id", sa.Integer, nullable=True)
+    )
+    op.add_column(
+        "series_page", sa.Column("stated_page", sa.Integer, nullable=True)
+    )
+
+    conn = op.get_bind()
+    # One auto-chapter per existing series (any series_tag_id present in pages).
+    conn.execute(
+        sa.text(
+            "INSERT INTO series_chapter "
+            "(series_tag_id, chapter_number, is_placeholder, created_at, updated_at) "
+            "SELECT DISTINCT series_tag_id, 1, false, now(), now() "
+            "FROM series_page"
+        )
+    )
+    # Point every existing page at its series' auto-chapter.
+    conn.execute(
+        sa.text(
+            "UPDATE series_page sp "
+            "SET chapter_id = sc.id "
+            "FROM series_chapter sc "
+            "WHERE sc.series_tag_id = sp.series_tag_id"
+        )
+    )
+
+    # Now lock chapter_id down: NOT NULL + FK (cascade) + index.
+    op.alter_column("series_page", "chapter_id", nullable=False)
+    op.create_foreign_key(
+        "fk_series_page_chapter_id",
+        "series_page",
+        "series_chapter",
+        ["chapter_id"],
+        ["id"],
+        ondelete="CASCADE",
+    )
+    op.create_index(
+        "ix_series_page_chapter_id", "series_page", ["chapter_id"]
+    )
+
+
+def downgrade() -> None:
+    op.drop_index("ix_series_page_chapter_id", table_name="series_page")
+    op.drop_constraint(
+        "fk_series_page_chapter_id", "series_page", type_="foreignkey"
+    )
+    op.drop_column("series_page", "stated_page")
+    op.drop_column("series_page", "chapter_id")
+    op.drop_index("ix_series_chapter_series_tag_id", table_name="series_chapter")
+    op.drop_table("series_chapter")