feat(backend): establish gin sqlite service skeleton

backend-api/SCAFFOLD.md

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+# Scaffold provenance
+
+- Generator: [Melkeydev/go-blueprint](https://github.com/Melkeydev/go-blueprint)
+- Version: `v0.10.11`
+- License: MIT
+- Reference command:
+  `go-blueprint create --name backend-api --framework gin --driver sqlite --git skip`
+
+The generator was run once outside this repository. Its current output targets Go 1.25 and
+Gin 1.12 and includes demo routes, permissive CORS, implicit `.env` loading, a package-level
+database singleton, and fatal health-check behavior. Those defaults were not copied. The checked-in
+code retains only the minimal command/internal package shape and is independently constrained by
+this project's architecture and tests.