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>

.dockerignore

@@ -0,0 +1,10 @@
+.git
+.gitignore
+.DS_Store
+naviwatcher
+naviwatcher-linux
+naviwatcher-mac
+naviwatcher.exe
+data/
+.claude/
+.ralphex/