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.yaml

@@ -43,14 +43,14 @@ definitions:
 host: localhost:8080
 info:
   contact:
-    email: support@simplebank.com
+    email: support@example.com
     name: API Support
-  description: API for Simple Bank application
+  description: API for API Template application
   license:
     name: MIT
     url: https://opensource.org/licenses/MIT
   termsOfService: http://swagger.io/terms/
-  title: Simple Bank API
+  title: API Template
   version: "1.0"
 paths:
   /v1/auth/register: