Enhance Makefile and documentation for validation workflow

- Added new targets in the Makefile for validation, including `run-validation`, `validate-check`, and `validate-kill`, to streamline the testing process with external tools like SteamPrefill.
- Introduced a `setcap` target to manage necessary capabilities for running the server on port 80 without root access.
- Updated README.md to include detailed instructions for validating functionality, including quick start guides and troubleshooting tips.
- Improved .gitignore to exclude validation artifacts and logs, ensuring a cleaner repository.

.gitignore

@@ -2,11 +2,19 @@
 /dist/
 /bin/
 steamcache2
+
+# Downloaded SteamPrefill client simulator (auto-managed by make client)
+/bin/steam-prefill/*
+!/bin/steam-prefill/.gitkeep
 /plans/
 
-#disk cache
+#validation artifacts
+/validate-disk/
 /disk/
 
+#logs
+*.log
+
 #config file
 /config.yaml