feat(buildinfo): add build metadata and expose via HTTP headers

Introduce a new `buildinfo` package to store version, commit, and build date metadata. Update HTTP handlers to include build metadata in response headers and modify initialization to set `buildinfo` values during runtime.

internal/buildinfo/buildinfo.go

@@ -0,0 +1,15 @@
+// Package buildinfo exposes compile-time metadata shared across the server.
+package buildinfo
+
+// The following variables are overridden via ldflags during release builds.
+// Defaults cover local development builds.
+var (
+	// Version is the semantic version or git describe output of the binary.
+	Version = "dev"
+
+	// Commit is the git commit SHA baked into the binary.
+	Commit = "none"
+
+	// BuildDate records when the binary was built in UTC.
+	BuildDate = "unknown"
+)