[feat] extend database schema with new tables and indexes

database/main/spell_modification.sql

@@ -0,0 +1,17 @@
+create table spell_modification
+(
+    id                  INTEGER
+        primary key,
+    spell_id            INTEGER not null
+        references spell
+            on delete cascade,
+    name                TEXT    not null,
+    wirkung             TEXT    not null,
+    voraussetzung       TEXT,
+    kosten_modifikation TEXT,
+    unique (spell_id, name)
+);
+
+create index idx_spell_mod_spell
+    on spell_modification (spell_id);
+