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/template_field.py

@@ -25,8 +25,8 @@ from sqlalchemy.orm import Mapped, mapped_column, relationship
 from app.db.base import Base
 
 if TYPE_CHECKING:  # pragma: no cover - typing only
-    from app.models.paper_template import PaperTemplate
-    from app.models.section_field import SectionField
+    from app.models.template import Template
+    from app.models.template_field_library import TemplateFieldLibrary
 
 
 class TemplateField(Base):
@@ -44,7 +44,7 @@ class TemplateField(Base):
     id: Mapped[int] = mapped_column(primary_key=True, autoincrement=True)
 
     template_id: Mapped[int] = mapped_column(
-        ForeignKey("paper_template.id", ondelete="CASCADE"),
+        ForeignKey("template.id", ondelete="CASCADE"),
         nullable=False,
     )
 
@@ -52,7 +52,7 @@ class TemplateField(Base):
     #: under it. The API enforces this in Python as well, since TiDB does not
     #: enforce foreign keys itself.
     field_id: Mapped[int] = mapped_column(
-        ForeignKey("section_field.id", ondelete="RESTRICT"),
+        ForeignKey("template_field_library.id", ondelete="RESTRICT"),
         nullable=False,
     )
 
@@ -60,11 +60,11 @@ class TemplateField(Base):
     #: sorts render first regardless of the field's ``level``.
     sort: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
 
-    template: Mapped["PaperTemplate"] = relationship(back_populates="items")
+    template: Mapped["Template"] = relationship(back_populates="items")
 
     #: Eager-loaded because every template read needs the field's name and
     #: typography; lazy loading would emit one query per row.
-    field: Mapped["SectionField"] = relationship(lazy="joined")
+    field: Mapped["TemplateFieldLibrary"] = relationship(lazy="joined")
 
     def __repr__(self) -> str:  # pragma: no cover - debugging aid
         return (