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/service/mock/service.go

@@ -1,9 +1,9 @@
 // Code generated by MockGen. DO NOT EDIT.
-// Source: github.com/FernandoJVideira/simple_bank/internal/service (interfaces: UserService,AuthService)
+// Source: github.com/FernandoJVideira/api_tmpl/internal/service (interfaces: UserService,AuthService)
 //
 // Generated by this command:
 //
-//	mockgen --package mocksvc --destination internal/service/mock/service.go github.com/FernandoJVideira/simple_bank/internal/service UserService,AuthService
+//	mockgen --package mocksvc --destination internal/service/mock/service.go github.com/FernandoJVideira/api_tmpl/internal/service UserService,AuthService
 //
 
 // Package mocksvc is a generated GoMock package.
@@ -13,8 +13,8 @@ import (
 	context "context"
 	reflect "reflect"
 
-	db "github.com/FernandoJVideira/simple_bank/internal/db/sqlc"
-	service "github.com/FernandoJVideira/simple_bank/internal/service"
+	db "github.com/FernandoJVideira/api_tmpl/internal/db/sqlc"
+	service "github.com/FernandoJVideira/api_tmpl/internal/service"
 	gomock "go.uber.org/mock/gomock"
 )