Add naming scheme selection to run configuration

Add a nullable naming_scheme column to NuzlockeRun so users can pick a
themed word category for nickname suggestions. Includes Alembic migration,
updated Pydantic schemas, a GET /runs/naming-categories endpoint backed by
a cached dictionary loader, and frontend dropdowns in both the NewRun
creation flow and the RunDashboard for mid-run changes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/app/models/nuzlocke_run.py

@@ -22,6 +22,7 @@ class NuzlockeRun(Base):
     )
     completed_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
     hof_encounter_ids: Mapped[list[int] | None] = mapped_column(JSONB, default=None)
+    naming_scheme: Mapped[str | None] = mapped_column(String(50), nullable=True)
 
     game: Mapped["Game"] = relationship(back_populates="runs")
     encounters: Mapped[list["Encounter"]] = relationship(back_populates="run")