Refactor migrations to introduce BaseMigration class

Switched from using Migration as the base class to a new abstract
BaseMigration class and parametrized Migration class to enforce
version specification at subclass definition. Updated various methods
and specs to utilize BaseMigration, improving type safety and
clarity of the migration process.

spec/migration_spec.cr

@@ -63,6 +63,6 @@ describe Cql::Migration do
   it "prints pending migrations" do
     migrator.down
     migrator.print_pending_migrations
-    migrator.pending_migrations.should be_a(Array(Cql::Migration.class))
+    migrator.pending_migrations.should be_a(Array(Cql::BaseMigration.class))
   end
 end

spec/migrations/alter_users_migration.cr

@@ -1,6 +1,4 @@
-class AlterUsersMigration < Cql::Migration
-  self.version = 987654321_i64
-
+class AlterUsersMigration < Cql::Migration(987654321)
   def up
     schema.alter :users do
       add_column :phone, String

spec/migrations/create_users_migration.cr

@@ -1,6 +1,4 @@
-class CreateUsersMigration < Cql::Migration
-  self.version = 123456789_i64
-
+class CreateUsersMigration < Cql::Migration(123456789)
   def up
     schema.users.create!
   end

src/migrations.cr

@@ -87,21 +87,23 @@ module Cql
   # migrator.last
   # ```
   #
-  abstract class Migration
-    getter schema : Schema
 
+  abstract class BaseMigration
+    abstract def up
+    abstract def down
+  end
+
+  abstract class Migration(V) < BaseMigration
     macro inherited
-      class_getter version : Int64 = 0_i64
-      def self.version=(number : Int64)
-        @@version = number
-      end
-      Cql::Migrator.migrations << {{@type}}
-    end
+      getter schema : Cql::Schema
 
-    def initialize(@schema : Schema); end
+      Cql::Migrator.migrations << {{@type}}
+      def self.version : Int32
+        V
+      end
 
-    abstract def up
-    abstract def down
+      def initialize(@schema : Cql::Schema); end
+    end
   end
 
   # The `Migrator` class is used to manage migrations and provides methods to apply,
@@ -142,22 +144,22 @@ module Cql
 
       getter id : Int64
       getter name : String
-      getter version : Int64
+      getter version : Int32
       getter created_at : Time
       getter updated_at : Time
 
       def initialize(
         @id : Int64,
         @name : String,
-        @version : Int64,
+        @version : Int32,
         @created_at = Time.local,
         @updated_at = Time.local
       )
       end
     end
 
     getter schema : Schema
-    class_property migrations : Array(Migration.class) = [] of Migration.class
+    class_property migrations : Array(BaseMigration.class) = [] of BaseMigration.class
     getter repo : Repository(MigrationRecord, Int32)
 
     def initialize(@schema : Schema)
@@ -224,7 +226,7 @@ module Cql
     # migrator.last
     # ```
     # @return [Migration.class | Nil]
-    def last : Migration.class | Nil
+    def last : BaseMigration.class | Nil
       Migrator.migrations.find { |m| m.version == repo.last.version }
     rescue DB::NoResultsError
       nil
@@ -259,7 +261,7 @@ module Cql
     # ```
     # migrator.print_rolled_back_migrations
     # ```
-    def print_rolled_back_migrations(m : Array(Migration.class))
+    def print_rolled_back_migrations(m : Array(BaseMigration.class))
       print_table(m.map { |migration| build_migration_record(migration) }, "✗".colorize.red.to_s)
     end
 
@@ -287,7 +289,7 @@ module Cql
     # ```
     # migrator.pending_migrations
     # ```
-    def pending_migrations : Array(Migration.class)
+    def pending_migrations : Array(BaseMigration.class)
       sorted_migrations.reject { |m| migration_applied?(m.version) }
     end
 
@@ -315,7 +317,7 @@ module Cql
       puts table
     end
 
-    private def build_migration_record(migration : Migration.class) : MigrationRecord
+    private def build_migration_record(migration : BaseMigration.class) : MigrationRecord
       MigrationRecord.new(0, migration.name, migration.version)
     end
 
@@ -327,7 +329,7 @@ module Cql
       schema.table :schema_migrations do
         primary :id, Int32
         column :name, String
-        column :version, Int64, index: true, unique: true
+        column :version, Int32, index: true, unique: true
         timestamps
       end
       schema.schema_migrations.create!
@@ -339,11 +341,11 @@ module Cql
       false
     end
 
-    private def record_migration(migration : Migration.class)
+    private def record_migration(migration : BaseMigration.class)
       repo.create(name: migration.name, version: migration.version)
     end
 
-    private def remove_migration_record(migration : Migration.class)
+    private def remove_migration_record(migration : BaseMigration.class)
       repo.delete_by(name: migration.name, version: migration.version)
     end
   end