chore: improve dev tooling (CI tests, dependabot, coverage) (#169)

- Add frontend unit tests with coverage to CI test workflow
- Add dependabot.yml for automated dependency updates (npm + GitHub Actions)
- Add backend coverage thresholds (60/65/50/60) to vitest.config.ts
- Exclude services/ and logger from coverage (untestable schedulers)

.github/dependabot.yml

@@ -0,0 +1,53 @@
+version: 2
+
+updates:
+  # Backend dependencies
+  - package-ecosystem: "npm"
+    directory: "/backend"
+    schedule:
+      interval: "weekly"
+      day: "monday"
+    open-pull-requests-limit: 10
+    labels:
+      - "dependencies"
+    groups:
+      minor-and-patch:
+        update-types:
+          - "minor"
+          - "patch"
+
+  # Frontend dependencies
+  - package-ecosystem: "npm"
+    directory: "/frontend"
+    schedule:
+      interval: "weekly"
+      day: "monday"
+    open-pull-requests-limit: 10
+    labels:
+      - "dependencies"
+    groups:
+      minor-and-patch:
+        update-types:
+          - "minor"
+          - "patch"
+
+  # Root dev dependencies
+  - package-ecosystem: "npm"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+      day: "monday"
+    open-pull-requests-limit: 5
+    labels:
+      - "dependencies"
+
+  # GitHub Actions
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+      day: "monday"
+    open-pull-requests-limit: 5
+    labels:
+      - "dependencies"
+      - "ci"