feat(internal): add foundational packages — config, logger, errors

- config: env-based loading with startup validation, 13 tests
- logger: structured slog wrapper with redaction, 10 tests
- errors: unified AppError model with HTTP mapping, 18 tests
- 41 tests total, build + vet clean

backend/go.mod

@@ -0,0 +1,5 @@
+module git.yeij.top/AskaEth/Cyrene_v2
+
+go 1.25.0
+
+require github.com/joho/godotenv v1.5.1