backend: add section-field and paper-template schema, API, and seed data

Three tables behind the template configuration feature:

  section_field   the reusable heading library (name, level, font size,
                  colour)
  paper_template  a named outline (name, abstract)
  template_field  the join, and the only home of display order (sort)

The field library is deliberately flat. A `parent_id` would tie a level-2
heading to exactly one level-1 heading, and the point of the feature is that
a field such as "Background" can sit under both "1. Introduction" and
"2. Related Work" — and in any number of templates, at a different position
in each. Hierarchy is expressed only by `level`, which is a rendering hint.

Display order lives entirely in `template_field.sort`. The order fields were
picked in is never stored, so selecting fields out of order and assigning
sorts renders in sort order. Two consequences are intentional and documented
on the model: repeats are allowed (no unique constraint on template+field)
and ties are legal (broken by insertion order, so the ordering is total).

Templates reference library fields rather than copying them, so renaming or
restyling a field updates every template that places it.

TiDB parses FOREIGN KEY and then ignores it, so the constraints are declared
for documentation and the integrity is enforced in the application layer:
deleting a field still in use is refused with the field names, creating a
template against a missing field is refused, and deleting a template removes
its join rows through the ORM's delete-orphan cascade.

Also normalises font_color to #RRGGBB on write (accepting rgb() and
shorthand) and emits font_size as a JSON number rather than pydantic's
default Decimal string.

scripts/seed.py is idempotent and fills the library with a standard academic
outline plus three starter templates.

backend/app/models/__init__.py

@@ -1,15 +1,18 @@
 """SQLAlchemy ORM models.
 
-This package is intentionally empty.
-
-No model classes exist yet, and no tables are created by this project at
-import time. The schema will be introduced through Alembic migrations:
-
-    alembic revision --autogenerate -m "describe the change"
-    alembic upgrade head
-
-When the first model is written, add it here (or in a submodule imported from
-here) so that ``Base.metadata`` — and therefore autogenerate — can see it.
+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.
 """
 
-__all__: list[str] = []
+from app.models.mixins import TimestampMixin
+from app.models.paper_template import PaperTemplate
+from app.models.section_field import SectionField
+from app.models.template_field import TemplateField
+
+__all__ = [
+    "PaperTemplate",
+    "SectionField",
+    "TemplateField",
+    "TimestampMixin",
+]