ci: add path filters to all workflow triggers

Each image only builds when its relevant source changes:
- backend: packages/backend/**, packages/types/**, Dockerfile.backend
- frontend: packages/frontend/**, packages/types/**, Dockerfile
- buttplug: packages/buttplug/**, Dockerfile.buttplug, nginx.buttplug.conf

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/docker-build-backend.yml

@@ -7,9 +7,17 @@ on:
       - develop
     tags:
       - "v*.*.*"
+    paths:
+      - "packages/backend/**"
+      - "packages/types/**"
+      - "Dockerfile.backend"
   pull_request:
     branches:
       - main
+    paths:
+      - "packages/backend/**"
+      - "packages/types/**"
+      - "Dockerfile.backend"
   workflow_dispatch:
 
 env: