Clarify Docker digest update group

.github/dependabot.yml

@@ -1,10 +1,17 @@
 version: 2
 updates:
   # Python dependencies
+  # Dependabot supports uv version updates, but GitHub currently lists uv
+  # security updates as "Not applicable"; daily checks keep uv.lock moving
+  # while repo-level Dependabot alerts/security updates cover supported ecosystems.
   - package-ecosystem: "uv"
     directory: "/"
     schedule:
-      interval: "weekly"
+      interval: "daily"
+      time: "05:00"
+      timezone: "Europe/London"
+    cooldown:
+      default-days: 3
     open-pull-requests-limit: 10
     groups:
       python-deps:
@@ -16,20 +23,25 @@ updates:
     directory: "/src/frontend"
     schedule:
       interval: "weekly"
+    cooldown:
+      default-days: 3
     open-pull-requests-limit: 10
     groups:
       npm-deps:
         patterns: ["*"]
         update-types: ["minor", "patch"]
 
-  # Dockerfile base images
+  # Dockerfile base image digests. When a tag stays the same, Dependabot titles
+  # can only show digest prefixes, so keep the group name explicit.
   - package-ecosystem: "docker"
     directory: "/"
     schedule:
       interval: "weekly"
+    cooldown:
+      default-days: 3
     open-pull-requests-limit: 5
     groups:
-      docker-images:
+      docker-base-image-digests:
         patterns: ["*"]
     ignore:
       # Node.js: block major-version bumps so dependabot never proposes
@@ -43,6 +55,8 @@ updates:
     directory: "/"
     schedule:
       interval: "weekly"
+    cooldown:
+      default-days: 3
     open-pull-requests-limit: 5
     groups:
       gh-actions: