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

docs/swagger/swagger-initializer.js

@@ -3,7 +3,7 @@ window.onload = function () {
 
   // the following lines will be replaced by docker/configurator, when it runs in a docker-container
   window.ui = SwaggerUIBundle({
-    url: "simple_bank.swagger.json",
+    url: "api_tmpl.swagger.json",
     dom_id: "#swagger-ui",
     deepLinking: true,
     presets: [SwaggerUIBundle.presets.apis, SwaggerUIStandalonePreset],