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/__init__.py

@@ -7,6 +7,22 @@ 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.
+
+Table naming
+------------
+Every table is prefixed with the module it belongs to, and a class is named
+after its table (``Template`` -> ``template``):
+
+    template                  the 模板 module
+    template_field            a field placed in a template, with its position
+    template_field_library    the 字段库 the template module draws on
+    paper                     the 论文 module
+    paper_sentence            one sentence of one paper
+    paper_sentence_reference  a citation of one sentence
+
+``template_field`` and ``template_field_library`` are one word apart and mean
+opposite things: the first is a *placement* (this template puts this field
+here, ``sort`` included), the second is the *catalogue* it was picked from.
 """
 
 from app.models.mixins import TimestampMixin
@@ -19,9 +35,9 @@ from app.models.paper import (
 )
 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 import Template
 from app.models.template_field import TemplateField
+from app.models.template_field_library import TemplateFieldLibrary
 
 __all__ = [
     "PAPER_STATUSES",
@@ -31,8 +47,8 @@ __all__ = [
     "Paper",
     "PaperSentence",
     "PaperSentenceReference",
-    "PaperTemplate",
-    "SectionField",
+    "Template",
     "TemplateField",
+    "TemplateFieldLibrary",
     "TimestampMixin",
 ]