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>

justfile

@@ -80,6 +80,9 @@ run:
 	@templ generate
 	@go run cmd/main.go
 
+build-run: build
+	@go run cmd/main.go
+
 test: build
 	@echo "Testing..."
 	@go test ./... -v