chore: add just lint and just fmt recipes

Add standalone lint and fmt (auto-fix) recipes to the justfile so
developers can check style without running the full build. Also
extract upload validation into a private Server helper to keep
the admin routes method under the complexity threshold.

justfile

@@ -19,6 +19,16 @@ dev:
 	@echo "→ Starting volumen (dev) on http://localhost:9090 …"
 	bundle exec exe/volumen serve --config ./config.toml --content ./posts
 
+# Lint the codebase (RuboCop, zero offenses)
+lint:
+	@echo "→ Linting…"
+	bundle exec rubocop
+
+# Auto-fix lint issues
+fmt:
+	@echo "→ Auto-fixing…"
+	bundle exec rubocop -A
+
 # Lint and package the gem (must pass with zero warnings)
 build:
 	@echo "→ Linting…"