feat(docker): add single-image multi-stage Docker setup with compose

- Multi-stage build: Node builds the frontend, Go builds a static
  CGO-free binary, runtime is Alpine with CA certificates
- Add docker-compose.yml with persistent volume for the SQLite database
- Add .dockerignore to keep the build context slim
- Embed time/tzdata in the Go binary for OS-independent timezones
- Document image build, startup, secrets, and reverse-proxy setup in README

backend/cmd/server/main.go

@@ -3,6 +3,8 @@ package main
 import (
 	"log"
 	"net/http"
+	// Zeitzonen-Datenbank einbetten – macht das Binary unabhängig vom OS (Docker/Distroless)
+	_ "time/tzdata"
 
 	"wannpassts/internal/calendar"
 	"wannpassts/internal/config"