chore: replace Renovate with Dependabot (#1651)

Switch from Renovate to GitHub's native Dependabot for dependency
updates.

Configuration:
- Weekly schedule with 3-day cooldown (matches npm's unpublish window)
- npm: covers root + all 4 workspace packages (db-service, sqlite,
postgres, hana)
- github-actions: single block at root
- Grouped PRs: minor+patch in one PR, majors separate, actions in one PR
- Ignored deps carried over from renovate.json: saplabs/hanaexpress,
amannn/action-semantic-pull-request,
martinbeentjes/npm-get-version-action

Note: Renovate's `lockFileMaintenance` (periodic lock-file-only
refreshes) has no Dependabot equivalent. Lock files are still updated on
every dep-bump PR.

- [ ] TODO: Uninstall the Renovate GitHub App from the repository
settings (Settings > GitHub Apps) to prevent it from continuing to open
PRs.

---------

Co-authored-by: Johannes Vogel <31311694+johannes-vogel@users.noreply.github.com>

Author: patricebender

.github/dependabot.yml

@@ -0,0 +1,39 @@
+version: 2
+updates:
+  - package-ecosystem: npm
+    directories:
+      - /
+      - /db-service
+      - /sqlite
+      - /postgres
+      - /hana
+    schedule:
+      interval: weekly
+    cooldown:
+      default-days: 3
+    groups:
+      dependencies:
+        patterns:
+          - "*"
+        update-types:
+          - minor
+          - patch
+      dependencies-major:
+        patterns:
+          - "*"
+        update-types:
+          - major
+
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: weekly
+    cooldown:
+      default-days: 3
+    ignore:
+      - dependency-name: saplabs/hanaexpress
+      - dependency-name: amannn/action-semantic-pull-request
+    groups:
+      github-actions:
+        patterns:
+          - "*"