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/docs.go

@@ -12,7 +12,7 @@ const docTemplate = `{
         "termsOfService": "http://swagger.io/terms/",
         "contact": {
             "name": "API Support",
-            "email": "support@simplebank.com"
+            "email": "support@example.com"
         },
         "license": {
             "name": "MIT",
@@ -159,8 +159,8 @@ var SwaggerInfo = &swag.Spec{
 	Host:             "localhost:8080",
 	BasePath:         "/v1",
 	Schemes:          []string{"http", "https"},
-	Title:            "Simple Bank API",
-	Description:      "API for Simple Bank application",
+	Title:            "API Template",
+	Description:      "Backend API template service",
 	InfoInstanceName: "swagger",
 	SwaggerTemplate:  docTemplate,
 	LeftDelim:        "{{",