chore: rename simple_bank to api_tmpl and clean up for template use

Rename the module, proto service, and all identifiers from simple_bank/
SimpleBank to api_tmpl/ApiTmpl, and regenerate the gRPC/gateway/swagger/
mock code accordingly.

- Scrub real secrets (SMTP password, auth token secret) from api.env/
  api_test.env, untrack them, and add api.env.example/api_test.env.example
- Remove committed build artifacts (tmp/main, tmp/build-errors.log,
  dbml-error.log) and gitignore them going forward
- Fix Dockerfile paths (build target, migrations copy) that referenced a
  stale project layout
- Rewrite README to match the actual cmd/ + internal/ structure and add a
  "using this as a template" section

internal/handlers/grpc/rpc_create_user_test.go

@@ -5,13 +5,13 @@ import (
 	"database/sql"
 	"testing"
 
-	db "github.com/FernandoJVideira/simple_bank/internal/db/sqlc"
-	"github.com/FernandoJVideira/simple_bank/internal/domain/role"
-	"github.com/FernandoJVideira/simple_bank/internal/security"
-	"github.com/FernandoJVideira/simple_bank/internal/service"
-	mocksvc "github.com/FernandoJVideira/simple_bank/internal/service/mock"
-	"github.com/FernandoJVideira/simple_bank/pb"
-	"github.com/FernandoJVideira/simple_bank/pkg/testutil"
+	db "github.com/FernandoJVideira/api_tmpl/internal/db/sqlc"
+	"github.com/FernandoJVideira/api_tmpl/internal/domain/role"
+	"github.com/FernandoJVideira/api_tmpl/internal/security"
+	"github.com/FernandoJVideira/api_tmpl/internal/service"
+	mocksvc "github.com/FernandoJVideira/api_tmpl/internal/service/mock"
+	"github.com/FernandoJVideira/api_tmpl/pb"
+	"github.com/FernandoJVideira/api_tmpl/pkg/testutil"
 	"github.com/stretchr/testify/require"
 	"go.uber.org/mock/gomock"
 	"google.golang.org/grpc/codes"