Repo spring cleaning (#746)

- Add CI workflow (pytest + frontend typecheck/tests) on PRs
- Add CodeQL static analysis for Python and JS/TS
- Add Dependabot for pip, npm, Docker, and GitHub Actions
- Tighten workflow permissions

.github/dependabot.yml

@@ -0,0 +1,29 @@
+version: 2
+updates:
+  # Python dependencies
+  - package-ecosystem: "pip"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 10
+
+  # Frontend npm dependencies
+  - package-ecosystem: "npm"
+    directory: "/src/frontend"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 10
+
+  # Dockerfile base images
+  - package-ecosystem: "docker"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 5
+
+  # GitHub Actions
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 5