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/crud/paper.py

@@ -224,7 +224,7 @@ def read(paper: Paper) -> PaperRead:
             else 0
         ),
     )
-    return PaperRead(**item.model_dump(), abstract=paper.abstract)
+    return PaperRead(**item.model_dump())
 
 
 # --- document assembly -------------------------------------------------------
@@ -425,7 +425,7 @@ def update(
 
     ``values`` holds only the keys the client actually sent — the route derives
     it from ``model_fields_set``, which is the only way to tell "clear the
-    abstract" apart from "leave it alone"; both arrive as ``None``.
+    author" apart from "leave it alone"; both arrive as ``None``.
 
     When the template changes, every sentence is re-stamped with the new
     template id. The sentences themselves keep their positions, which is what