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

proto/user.proto

@@ -4,7 +4,7 @@ package pb;
 
 import "google/protobuf/timestamp.proto";
 
-option go_package = "github.com/FernandoJVideira/simple_bank/pb";
+option go_package = "github.com/FernandoJVideira/api_tmpl/pb";
 
 message User {
     string username = 1;