fix: add permissions to backend caller workflows

Reusable workflows inherit permissions from the caller. Without
explicit permissions in the callers, the nested jobs requesting
packages:write and contents:write are denied.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Junyi Hou

.github/workflows/build-backend-dev.yml

@@ -4,6 +4,10 @@ on:
     branches:
       - development
 
+permissions:
+  contents: write
+  packages: write
+
 jobs:
   build-backend-and-push:
     runs-on: ubuntu-latest

.github/workflows/build-backend-prd.yml

@@ -6,6 +6,10 @@ on:
   repository_dispatch:
     types: [prd]
 
+permissions:
+  contents: write
+  packages: write
+
 jobs:
   build-backend-and-push:
     runs-on: ubuntu-latest

.github/workflows/build-backend-stg.yml

@@ -6,6 +6,10 @@ on:
   repository_dispatch:
     types: [stg]
 
+permissions:
+  contents: write
+  packages: write
+
 jobs:
   build-backend-and-push:
     runs-on: ubuntu-latest