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/api/router.py

@@ -2,9 +2,10 @@
 
 from fastapi import APIRouter
 
-from app.api.routes import health, section_fields, templates
+from app.api.routes import health, papers, section_fields, templates
 
 api_router = APIRouter()
 api_router.include_router(health.router)
+api_router.include_router(papers.router)
 api_router.include_router(section_fields.router)
 api_router.include_router(templates.router)