Refactor handlers and update changelog for 5.0.0-Beta

- Split IndexHandler into HealthHandler, VersionHandler, and CharacterHandler
- Rename index.go to version.go in backend
- Change VersionData.Changelog from string to []string
- Add changelog entries for issues #16-#23
- Remove TestDB function and related code
- Fix import ordering in several files

Closes #21, #22
References #16, #17, #18, #19, #20, #23

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

internal/server/health_handler_test.go

@@ -0,0 +1,29 @@
+package server
+
+import (
+	"encoding/json"
+	"net/http"
+	"testing"
+
+	"music-server/internal/db"
+
+	"github.com/stretchr/testify/assert"
+)
+
+// TestHealthCheck verifies the health endpoint returns database status
+func TestHealthCheck(t *testing.T) {
+	// Setup database
+	db.TestSetupDB(t)
+	defer db.TestTearDownDB(t)
+
+	e := StartTestServer(t)
+
+	resp := MakeTestRequest(t, e, "GET", "/health")
+	assert.Equal(t, http.StatusOK, resp.Code)
+
+	var healthData map[string]string
+	err := json.Unmarshal(resp.Body.Bytes(), &healthData)
+	assert.NoError(t, err)
+	assert.NotEmpty(t, healthData)
+	assert.Equal(t, "up", healthData["status"])
+}