fix: auto-approval and refine dependabot policy (#412)

## What changed

This updates our Dependabot policy to reduce routine dependency-update
noise while keeping minor and patch updates moving automatically.

- Configure Dependabot to run weekly on Tuesday at 09:00 Europe/Paris
for both `github-actions` and `bun`
- Group all minor and patch updates per ecosystem:
  - one GitHub Actions update PR
  - one Bun dependency update PR
- Keep major updates ungrouped so Dependabot opens individual PRs for
manual review
- Reduce routine open Dependabot PRs to one per ecosystem
- Add cooldown windows so Dependabot avoids immediately chasing fresh
releases:
  - 7 days for minor updates
  - 2 days for patch updates
- Update the Dependabot automerge workflow to generate a GitHub App
token before approving PRs
- Auto-approve and enable automerge only for patch and minor updates,
including `0.x` minors
- Leave major update PRs for human review and merge

## Why

Dependabot was not able to approve/automerge PRs using the default
token. This follows the GitHub App token pattern recommended by
security, while also tuning Dependabot for a better signal-to-noise
ratio.

The resulting behavior is:

- minor/patch updates are batched weekly and can merge after CI passes
- major updates still appear, but individually and without automerge
- security updates remain handled by Dependabot/GitHub outside the
routine grouping policy