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/healthHandler.go

@@ -0,0 +1,28 @@
+package server
+
+import (
+	"net/http"
+
+	"github.com/labstack/echo/v5"
+	"music-server/internal/db"
+)
+
+type HealthHandler struct {
+}
+
+func NewHealthHandler() *HealthHandler {
+	return &HealthHandler{}
+}
+
+// HealthCheck godoc
+//
+//	@Summary		Check server health
+//	@Description	Returns the health status of the server
+//	@Tags			health
+//	@Accept			json
+//	@Produce		json
+//	@Success		200	{string}	string	"OK"
+//	@Router			/health [get]
+func (h *HealthHandler) HealthCheck(ctx *echo.Context) error {
+	return ctx.JSON(http.StatusOK, db.Health())
+}