Add .dockerignore - keep local data and venv out of the image

.dockerignore

@@ -0,0 +1,30 @@
+.git
+.gitignore
+.claude-context
+venv/
+env/
+ENV/
+__pycache__/
+*.py[cod]
+*.egg-info/
+
+# Local data must never bake into the image
+*.db
+*.sqlite
+*.sqlite3
+app/static/uploads/
+backups/
+*.zip
+*.zip.part
+.env
+
+# Windows-only helpers
+*.bat
+
+# Docs and scratch
+docs/
+*.md
+!README.md
+tests/
+.vscode/
+.idea/