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

pb/rpc_create_user.pb.go

@@ -1,7 +1,7 @@
 // Code generated by protoc-gen-go. DO NOT EDIT.
 // versions:
 // 	protoc-gen-go v1.36.10
-// 	protoc        v6.33.1
+// 	protoc        v7.36.0
 // source: rpc_create_user.proto
 
 package pb
@@ -145,7 +145,7 @@ const file_rpc_create_user_proto_rawDesc = "" +
 	"\x05email\x18\x03 \x01(\tR\x05email\x12\x1a\n" +
 	"\bpassword\x18\x04 \x01(\tR\bpassword\"2\n" +
 	"\x12CreateUserResponse\x12\x1c\n" +
-	"\x04user\x18\x01 \x01(\v2\b.pb.UserR\x04userB,Z*github.com/FernandoJVideira/simple_bank/pbb\x06proto3"
+	"\x04user\x18\x01 \x01(\v2\b.pb.UserR\x04userB)Z'github.com/FernandoJVideira/api_tmpl/pbb\x06proto3"
 
 var (
 	file_rpc_create_user_proto_rawDescOnce sync.Once