docs(project): add reference Python/Flask implementation

Previous version of Tanabata used as visual and functional reference
for the new Go + SvelteKit rewrite.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/reference/backend/go.mod

@@ -0,0 +1,19 @@
+module tanabata
+
+go 1.23.0
+
+toolchain go1.23.10
+
+require github.com/jackc/pgx/v5 v5.7.5
+
+require (
+	github.com/jackc/pgpassfile v1.0.0 // indirect
+	github.com/jackc/pgservicefile v0.0.0-20240606120523-5a60cdf6a761 // indirect
+	github.com/jackc/pgx v3.6.2+incompatible // indirect
+	github.com/jackc/puddle/v2 v2.2.2 // indirect
+	github.com/pkg/errors v0.9.1 // indirect
+	github.com/stretchr/testify v1.9.0 // indirect
+	golang.org/x/crypto v0.37.0 // indirect
+	golang.org/x/sync v0.13.0 // indirect
+	golang.org/x/text v0.24.0 // indirect
+)