ci: optimize workflows to cut redundant runs (#465)

* ci: optimize workflows to cut redundant runs and add timeout protection

Skip validation jobs (build, test, lint, Docker, E2E) on push-to-main
since code was already validated on the PR. Skip CI entirely for
release-please PRs (version bump only). Add timeout-minutes to all jobs
to prevent hung runners from burning 360 min (GitHub default). Add npm
caching to the start-app composite action. Add concurrency control to
docker-publish. Group Dependabot updates to batch PRs per ecosystem.

Estimated savings: ~10 min per release cycle, ~30-40 min/day.

* chore: add tsbuildinfo to gitignore

.github/dependabot.yml

@@ -7,6 +7,9 @@ updates:
     labels:
       - "dependencies"
     open-pull-requests-limit: 10
+    groups:
+      all-npm-root:
+        patterns: ["*"]
 
   - package-ecosystem: "npm"
     directory: "/backend"
@@ -15,6 +18,9 @@ updates:
     labels:
       - "dependencies"
     open-pull-requests-limit: 10
+    groups:
+      all-npm-backend:
+        patterns: ["*"]
 
   - package-ecosystem: "npm"
     directory: "/frontend"
@@ -23,6 +29,9 @@ updates:
     labels:
       - "dependencies"
     open-pull-requests-limit: 10
+    groups:
+      all-npm-frontend:
+        patterns: ["*"]
 
   - package-ecosystem: "github-actions"
     directory: "/"
@@ -30,3 +39,6 @@ updates:
       interval: "weekly"
     labels:
       - "ci"
+    groups:
+      all-actions:
+        patterns: ["*"]