backend: add the paper authoring schema, API, and smoke test

A paper is a document written against a template. Its structure is never
copied into it: `paper` points at a template and the outline is read live on
every render, so switching `template_id` re-shapes the whole document in one
write.

Sentences are addressed by *position* rather than by a template row id:
`paper_sentence.paper_template_filed_sort` holds the sort of the placement the
sentence belongs to, and `sort` holds its place inside that paragraph. That
indirection is what makes a template switch non-destructive — a sentence that
remembers "position 7" lands on whatever the new template puts at position 7 —
and it is why a paragraph is any position either the template or the content
mentions: the structure survives with no content, and content whose position
the template does not define is still rendered, in order, under 未设定.

Citations are a table rather than a column, since one sentence may quote
several references. `quote` is required — a citation that does not say what it
quotes is refused with 422 — while `reference_id` is a plain nullable integer
with no foreign key, because the reference library does not exist yet.

Deleting a template a paper is written against is refused with 409 and a count,
matching how the field library refuses to drop a field still in use.

scripts/smoke_papers.py walks the whole loop — create, empty structure, write a
paragraph with citations, switch templates, keep unmatched positions, move a
paragraph, delete — in 40 checks, and cleans up after itself.

backend/app/schemas/__init__.py

@@ -6,6 +6,23 @@ from app.schemas.common import (
     PageResult,
     normalize_hex_color,
 )
+from app.schemas.paper import (
+    CitationInput,
+    CitationRead,
+    PaperCitationRead,
+    PaperCreate,
+    PaperDocumentRead,
+    PaperListItem,
+    PaperRead,
+    PaperStatus,
+    PaperUpdate,
+    ParagraphListRead,
+    ParagraphRead,
+    ParagraphUpdate,
+    SentenceCreate,
+    SentenceRead,
+    SentenceUpdate,
+)
 from app.schemas.paper_template import (
     PaperTemplateCreate,
     PaperTemplateListItem,
@@ -23,6 +40,21 @@ from app.schemas.section_field import (
 
 __all__ = [
     "BatchDeleteRequest",
+    "CitationInput",
+    "CitationRead",
+    "PaperCitationRead",
+    "PaperCreate",
+    "PaperDocumentRead",
+    "PaperListItem",
+    "PaperRead",
+    "PaperStatus",
+    "PaperUpdate",
+    "ParagraphListRead",
+    "ParagraphRead",
+    "ParagraphUpdate",
+    "SentenceCreate",
+    "SentenceRead",
+    "SentenceUpdate",
     "BatchDeleteResult",
     "PageResult",
     "PaperTemplateCreate",