Add --prune flag to seed command to remove stale data

Without --prune, seeds continue to only upsert (add/update).
With --prune, routes, encounters, and bosses not present in the
seed JSON files are deleted from the database.

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

backend/src/app/seeds/run.py

@@ -38,9 +38,12 @@ def load_json(filename: str):
         return json.load(f)
 
 
-async def seed():
-    """Run the full seed process."""
-    print("Starting seed...")
+async def seed(*, prune: bool = False):
+    """Run the full seed process.
+
+    When prune is True, removes DB rows not present in seed data.
+    """
+    print("Starting seed..." + (" (with pruning)" if prune else ""))
 
     async with async_session() as session, session.begin():
         # 1. Upsert version groups
@@ -88,7 +91,7 @@ async def seed():
                 continue
 
             # Upsert routes once per version group
-            route_map = await upsert_routes(session, vg_id, routes_data)
+            route_map = await upsert_routes(session, vg_id, routes_data, prune=prune)
             route_maps_by_vg[vg_id] = route_map
             total_routes += len(route_map)
             print(f"  {vg_slug}: {len(route_map)} routes")
@@ -119,6 +122,7 @@ async def seed():
                             route["encounters"],
                             dex_to_id,
                             game_id,
+                            prune=prune,
                         )
                         total_encounters += enc_count
 
@@ -137,6 +141,7 @@ async def seed():
                             child["encounters"],
                             dex_to_id,
                             game_id,
+                            prune=prune,
                         )
                         total_encounters += enc_count
 
@@ -160,7 +165,13 @@ async def seed():
 
             route_name_to_id = route_maps_by_vg.get(vg_id, {})
             boss_count = await upsert_bosses(
-                session, vg_id, bosses_data, dex_to_id, route_name_to_id, slug_to_id
+                session,
+                vg_id,
+                bosses_data,
+                dex_to_id,
+                route_name_to_id,
+                slug_to_id,
+                prune=prune,
             )
             total_bosses += boss_count
             print(f"  {vg_slug}: {boss_count} bosses")