feat: add Docker setup with multi-stage build and docker-compose

- Add Dockerfile with golang:1.25-alpine build stage + alpine runtime stage
- Add docker-compose.yml with naviwatcher service and data volume
- Add .dockerignore for build context cleanup
- Add .gitignore for build artifacts and data directory
- Update plan: mark Task 7 checkboxes as complete

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -0,0 +1,11 @@
+services:
+  naviwatcher:
+    build:
+      context: .
+      dockerfile: Dockerfile
+    container_name: naviwatcher
+    ports:
+      - "8080:8080"
+    volumes:
+      - ./data:/app/data
+    restart: unless-stopped