refactor: prefix every table with its module

Two tables were named after the concept they came from rather than the module
they belong to, so the schema read as if the template tables were part of the
paper module. Renamed (data preserved, `RENAME TABLE` moves rows in place):

    paper_template  -> template                 the 模板 module
    section_field   -> template_field_library   the 字段库 the 模板 module owns

The paper tables and `template_field` already followed the rule. The rename
carries through everything that named a module:

  models   Template, TemplateField, TemplateFieldLibrary
  schemas  Template*, TemplateFieldLibrary*
  crud     app/crud/template.py, app/crud/template_field_library.py
  API      /template-field-library (was /section-fields); handlers are now
           named after library entries, which removes the ambiguity with
           TemplateField — a placement, a different thing entirely
  client   src/api/templateFieldLibrary.ts

`paper_template_filed_sort` is deliberately untouched: it is a column of the
paper module, spelled as the feature was specified.

TiDB v8.5 with tidb_enable_foreign_key on — as this cluster runs — enforces
foreign keys rather than ignoring them, so the docs' "TiDB does not enforce
foreign keys" was wrong. Corrected, with what actually follows from it: the
rename was rehearsed (RENAME TABLE carries a referencing constraint along), the
API keeps checking first so a violation names the row instead of surfacing a
driver error, and the ORM cascades stay so behaviour does not depend on a
cluster setting.

Revision f27a1c6d9e04 verified both ways; 40 smoke checks, type-check and build
all pass.

backend/app/models/paper.py

@@ -3,7 +3,7 @@
 A paper is a bag of metadata plus two things that are deliberately kept apart:
 
 * its **structure**, which is not stored here at all. Every render reads the
-  :class:`~app.models.paper_template.PaperTemplate` the paper points at, live.
+  :class:`~app.models.template.Template` the paper points at, live.
   Nothing is copied, so switching ``template_id`` re-shapes the whole document
   in one write.
 * its **content**, which lives in
@@ -33,7 +33,7 @@ from app.models.mixins import TimestampMixin
 
 if TYPE_CHECKING:  # pragma: no cover - typing only
     from app.models.paper_sentence import PaperSentence
-    from app.models.paper_template import PaperTemplate
+    from app.models.template import Template
 
 #: Writing state of a paper: 草稿 / 撰写中 / 已完成.
 #:
@@ -65,7 +65,7 @@ class Paper(TimestampMixin, Base):
     #: ``RESTRICT`` documents the intent (a template in use must not vanish);
     #: TiDB does not enforce it, so the API refuses the template delete too.
     template_id: Mapped[int | None] = mapped_column(
-        ForeignKey("paper_template.id", ondelete="RESTRICT"),
+        ForeignKey("template.id", ondelete="RESTRICT"),
         nullable=True,
     )
 
@@ -93,7 +93,7 @@ class Paper(TimestampMixin, Base):
 
     #: 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["PaperTemplate | None"] = relationship(lazy="joined")
+    template: Mapped["Template | None"] = relationship(lazy="joined")
 
     #: The paper's sentences, in document order.
     #: