refactor: make the abstract a paragraph instead of a paper field

A template's abstract paragraph (`0 Abstract`) *is* the paper's abstract: it has
a position in the document, the heading and typography the template gives it,
and the same sentence-by-sentence editing as everything else. `paper.abstract`
was a second home for that text — one the document never reads, so a paper
could show two different abstracts and the column could drift from the body.

The column is gone, from the table, the model, the schemas, the API payloads,
the paper form, the list subtitle and the paper page. Nothing else changed.

The text already written into it is not gone. Revision a83f5c21d7b6 writes each
stored abstract into the paper's body first — one sentence per 。 at the
paragraph carrying the abstract heading, appended after anything already there
rather than replacing it. A template without such a heading keeps the text too,
one position above its first paragraph, where the document renders it under
未设定. Verified against the one paper that had an abstract: 450 characters in,
450 out, identical including `|J| ≤ α · I⁻ᵝ`, split into six sentences in the
`0 Abstract` paragraph, still with its own edit button.

The smoke test no longer assumes an empty database: it records the paper total
before it starts and compares against that, so it can run on a real one.

backend/app/models/paper.py

@@ -25,7 +25,7 @@ without a migration.
 
 from typing import TYPE_CHECKING
 
-from sqlalchemy import ForeignKey, Integer, String, Text, text
+from sqlalchemy import ForeignKey, Integer, String, text
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 
 from app.db.base import Base
@@ -69,9 +69,6 @@ class Paper(TimestampMixin, Base):
         nullable=True,
     )
 
-    #: 摘要 — the paper's own abstract, free text.
-    abstract: Mapped[str | None] = mapped_column(Text, nullable=True)
-
     #: 作者. Free text, since authorship is written as it will be printed.
     author: Mapped[str | None] = mapped_column(String(255), nullable=True)
 
@@ -91,6 +88,12 @@ class Paper(TimestampMixin, Base):
     #: 投稿目标期刊.
     target_journal: Mapped[str | None] = mapped_column(String(255), nullable=True)
 
+    #: There is deliberately no ``abstract`` column. A paper's abstract is a
+    #: paragraph of its body — the template's ``0 Abstract`` field — so it has a
+    #: position in the document, the template's own heading, and the same
+    #: sentence-by-sentence editing as everything else. A column here would be a
+    #: second home for the same text, and the document never reads it.
+
     #: Eager-loaded with the paper: every read of a paper shows its template
     #: name, and a lazy load there would be one query per row in the table.
     template: Mapped["Template | None"] = relationship(lazy="joined")