Add PokeDB sprite downloading (100x100 WebP)

Download pokemon sprites from PokeDB CDN during import, cached locally
as {pokeapi_id}.webp. Replaces PokeAPI GitHub sprite URLs. ~4.6MB for
all 1119 unique sprites.

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

tools/import-pokedb/import_pokedb/__main__.py

@@ -19,6 +19,7 @@ from pathlib import Path
 from .loader import load_pokedb_data, load_seed_config
 from .mappings import PokemonMapper, LocationMapper, build_version_map, map_encounter_method
 from .processing import filter_encounters_for_game, process_encounters, build_routes
+from .sprites import download_sprites
 
 SEEDS_DIR_CANDIDATES = [
     Path("backend/src/app/seeds"),                # from repo root
@@ -131,8 +132,11 @@ def main(argv: list[str] | None = None) -> None:
             print(f"    - {m}", file=sys.stderr)
 
     # Spot-check pokemon mapping on actual encounter data
-    form_ids_in_encounters = {e.get("pokemon_form_identifier", "") for e in pokedb.encounters}
-    form_ids_in_encounters.discard("")
+    form_ids_in_encounters: set[str] = set()
+    for e in pokedb.encounters:
+        fid = e.get("pokemon_form_identifier")
+        if fid:
+            form_ids_in_encounters.add(fid)
     mapped_forms = 0
     for fid in form_ids_in_encounters:
         if pokemon_mapper.lookup(fid) is not None:
@@ -182,6 +186,12 @@ def main(argv: list[str] | None = None) -> None:
         print(f"  Routes: {total_routes}")
         print(f"  Encounter entries: {total_enc}")
 
+    # Download sprites for all encountered pokemon
+    print("\nDownloading sprites...")
+    sprites_dir = output_dir / "sprites"
+    sprite_map = download_sprites(pokemon_mapper, form_ids_in_encounters, sprites_dir)
+    print(f"  Sprite map covers {len(sprite_map)} forms")
+
     print("\nProcessing complete. Output not yet written (subtask gkcy).")