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>

.woodpecker.yml

@@ -0,0 +1,30 @@
+when:
+  branch: main
+  event: push
+
+steps:
+  - name: build-backend
+    image: woodpeckerci/plugin-docker-buildx
+    settings:
+      repo: git.mikhailkilin.ru/killingdruid/bbb-backend
+      registry: git.mikhailkilin.ru
+      dockerfile: backend/Dockerfile
+      context: backend
+      tags: latest
+      username:
+        from_secret: gitea_username
+      password:
+        from_secret: gitea_password
+
+  - name: build-frontend
+    image: woodpeckerci/plugin-docker-buildx
+    settings:
+      repo: git.mikhailkilin.ru/killingdruid/bbb-frontend
+      registry: git.mikhailkilin.ru
+      dockerfile: frontend/Dockerfile
+      context: frontend
+      tags: latest
+      username:
+        from_secret: gitea_username
+      password:
+        from_secret: gitea_password