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>

backend/src/app/seeds/__main__.py

@@ -1,20 +1,26 @@
 """Entry point for running seeds.
 
 Usage:
-    python -m app.seeds           # Run seed
-    python -m app.seeds --verify  # Run seed + verification
+    python -m app.seeds                  # Run seed
+    python -m app.seeds --verify         # Run seed + verification
+    python -m app.seeds --export-bosses  # Export boss data to seed JSON files
 """
 
 import asyncio
 import sys
 
 from app.core.database import engine
-from app.seeds.run import seed, verify
+from app.seeds.run import export_bosses, seed, verify
 
 
 async def main():
     verbose = "--verbose" in sys.argv or "-v" in sys.argv
     engine.echo = verbose
+
+    if "--export-bosses" in sys.argv:
+        await export_bosses()
+        return
+
     await seed()
     if "--verify" in sys.argv:
         await verify()