Separate PokeAPI ID from national dex for correct form identification

Pokemon forms (e.g., Alolan Rattata) had their PokeAPI ID (10091) stored as
national_dex, causing them to display incorrectly. This renames the unique
identifier to pokeapi_id and adds a real national_dex field shared between
forms and their base species, so Alolan Rattata correctly shows as #19.

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

backend/src/app/seeds/data/alpha-sapphire.json

@@ -4,7 +4,7 @@
     "order": 1,
     "encounters": [
       {
-        "national_dex": 252,
+        "pokeapi_id": 252,
         "pokemon_name": "treecko",
         "method": "starter",
         "encounter_rate": 100,
@@ -12,7 +12,7 @@
         "max_level": 5
       },
       {
-        "national_dex": 255,
+        "pokeapi_id": 255,
         "pokemon_name": "torchic",
         "method": "starter",
         "encounter_rate": 100,
@@ -20,7 +20,7 @@
         "max_level": 5
       },
       {
-        "national_dex": 258,
+        "pokeapi_id": 258,
         "pokemon_name": "mudkip",
         "method": "starter",
         "encounter_rate": 100,
@@ -34,7 +34,7 @@
     "order": 2,
     "encounters": [
       {
-        "national_dex": 345,
+        "pokeapi_id": 345,
         "pokemon_name": "lileep",
         "method": "fossil",
         "encounter_rate": 100,
@@ -42,7 +42,7 @@
         "max_level": 20
       },
       {
-        "national_dex": 347,
+        "pokeapi_id": 347,
         "pokemon_name": "anorith",
         "method": "fossil",
         "encounter_rate": 100,
@@ -56,7 +56,7 @@
     "order": 3,
     "encounters": [
       {
-        "national_dex": 360,
+        "pokeapi_id": 360,
         "pokemon_name": "wynaut",
         "method": "gift",
         "encounter_rate": 100,
@@ -70,7 +70,7 @@
     "order": 4,
     "encounters": [
       {
-        "national_dex": 351,
+        "pokeapi_id": 351,
         "pokemon_name": "castform",
         "method": "gift",
         "encounter_rate": 100,
@@ -84,7 +84,7 @@
     "order": 5,
     "encounters": [
       {
-        "national_dex": 374,
+        "pokeapi_id": 374,
         "pokemon_name": "beldum",
         "method": "gift",
         "encounter_rate": 100,