Add k3s deployment: Dockerfiles, nginx config, CI pipeline

- Backend Dockerfile: multi-stage rust:1.87 → debian:bookworm-slim
- Frontend Dockerfile: multi-stage node:22-alpine → nginx:alpine
- Frontend nginx.conf: serve static + proxy /api/ to backend service
- Change API_URL to relative /api for containerized deployment
- Woodpecker CI pipeline to build and push images to Gitea Registry
- Add repository URL to CLAUDE.md

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/api.ts

@@ -1,4 +1,4 @@
-const API_URL = "http://localhost:3000/api";
+const API_URL = "/api";
 
 export interface EntryMeta {
   id: number;