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>

backend/Dockerfile

@@ -0,0 +1,19 @@
+FROM rust:1.87 AS builder
+
+WORKDIR /app
+COPY Cargo.toml Cargo.lock* ./
+COPY src/ src/
+
+RUN cargo build --release
+
+FROM debian:bookworm-slim
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    libssl3 ca-certificates \
+    && rm -rf /var/lib/apt/lists/*
+
+COPY --from=builder /app/target/release/backend /usr/local/bin/backend
+
+EXPOSE 3000
+
+CMD ["backend"]