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_sentence.py

@@ -10,7 +10,7 @@ whole design:
     meaningfully have in common. Swap the paper's template and this sentence
     lands on whatever the new template puts at that position, with no per
     sentence editing. (The name keeps the spelling the feature was specified
-    with; it reads ``paper_template_field_sort``.)
+    with; it reads ``template_field_sort``.)
 
 ``sort``
     Where this sentence sits *inside* that paragraph. A paragraph is reassembled
@@ -40,7 +40,7 @@ from app.models.mixins import TimestampMixin
 if TYPE_CHECKING:  # pragma: no cover - typing only
     from app.models.paper import Paper
     from app.models.paper_sentence_reference import PaperSentenceReference
-    from app.models.paper_template import PaperTemplate
+    from app.models.template import Template
 
 
 class PaperSentence(TimestampMixin, Base):
@@ -70,7 +70,7 @@ class PaperSentence(TimestampMixin, Base):
     #: The template this sentence was written against — provenance only. Reads
     #: never filter on it; see the module docstring.
     template_id: Mapped[int | None] = mapped_column(
-        ForeignKey("paper_template.id", ondelete="SET NULL"),
+        ForeignKey("template.id", ondelete="SET NULL"),
         nullable=True,
     )