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/models/__init__.py

@@ -3,14 +3,34 @@
 Importing this package registers every model on ``Base.metadata``, which is
 what ``alembic revision --autogenerate`` inspects. A new model therefore has to
 be added to the imports below, not only to its own module.
+
+The import order here is for readability only: relationships are declared by
+name and resolved through the SQLAlchemy registry after every module has been
+imported, so a cycle between two model modules is not a problem.
 """
 
 from app.models.mixins import TimestampMixin
+from app.models.paper import (
+    PAPER_STATUSES,
+    STATUS_DONE,
+    STATUS_DRAFT,
+    STATUS_WRITING,
+    Paper,
+)
+from app.models.paper_sentence import PaperSentence
+from app.models.paper_sentence_reference import PaperSentenceReference
 from app.models.paper_template import PaperTemplate
 from app.models.section_field import SectionField
 from app.models.template_field import TemplateField
 
 __all__ = [
+    "PAPER_STATUSES",
+    "STATUS_DONE",
+    "STATUS_DRAFT",
+    "STATUS_WRITING",
+    "Paper",
+    "PaperSentence",
+    "PaperSentenceReference",
     "PaperTemplate",
     "SectionField",
     "TemplateField",