Add boss seed data pipeline for export and import

Add seeder support for boss battles so new database instances come
pre-populated. Adds --export-bosses CLI flag to dump boss data from the
database to JSON seed files, and loads those files during normal seeding.

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

.beans/nuzlocke-tracker-29oy--boss-seed-data-pipeline.md

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+---
+# nuzlocke-tracker-29oy
+title: Boss seed data pipeline
+status: completed
+type: feature
+priority: normal
+created_at: 2026-02-08T11:33:33Z
+updated_at: 2026-02-08T11:35:13Z
+---
+
+Export boss data from admin, save as seed JSON files, and load them during seeding.
+
+## Checklist
+- [x] Add unique constraint to boss_battles (version_group_id, order) + Alembic migration
+- [x] Add upsert_bosses to seed loader (loader.py)
+- [x] Add boss loading step to seed runner (run.py)
+- [x] Add boss count to verify() function
+- [x] Add export_bosses() function to run.py
+- [x] Add --export-bosses flag to __main__.py