Add 1-month cooldown to all Dependabot update configurations (#1656)

Dependabot was configured to create PRs immediately after new versions
were published. Adding a 30-day cooldown across all ecosystems reduces
churn from short-lived or unstable releases.

## Changes

- Added `cooldown: default-days: 30` to all five package ecosystems in
`.github/dependabot.yml`:
  - `uv` (backend)
  - `npm` (frontend, docs)
  - `github-actions`
  - `docker`
  - `docker-compose`

```yaml
- package-ecosystem: "uv"
  directory: "/backend"
  schedule:
    interval: "weekly"
  cooldown:
    default-days: 30
```

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: evroon <11857441+evroon@users.noreply.github.com>

.github/dependabot.yml

@@ -7,6 +7,8 @@ updates:
     directory: "/backend"
     schedule:
       interval: "weekly"
+    cooldown:
+      default-days: 30
     reviewers:
       - "evroon"
     ignore:
@@ -19,6 +21,8 @@ updates:
       - "/docs"
     schedule:
       interval: "weekly"
+    cooldown:
+      default-days: 30
     reviewers:
       - "evroon"
     ignore:
@@ -29,6 +33,8 @@ updates:
     directory: "/"
     schedule:
       interval: "weekly"
+    cooldown:
+      default-days: 30
     reviewers:
       - "evroon"
     ignore:
@@ -42,6 +48,8 @@ updates:
       - "/"
     schedule:
       interval: "weekly"
+    cooldown:
+      default-days: 30
     reviewers:
       - "evroon"
     ignore:
@@ -52,6 +60,8 @@ updates:
     directory: "/"
     schedule:
       interval: "weekly"
+    cooldown:
+      default-days: 30
     reviewers:
       - "evroon"
     ignore: