fix(k8s): migrate backend from Bun to Node.js and fix registry URLs

Backend Dockerfile changes:
- Replace Bun base image with node:20-alpine for production stability
- Change bun install to npm ci for dependency installation
- Update health check from Bun fetch to curl command
- Change CMD from 'bun server.js' to 'node server.js'

Deployment manifest changes:
- Update backend image URL to gitea-gitea-http.taildb3494.ts.net
- Update frontend image URL to gitea-gitea-http.taildb3494.ts.net
- Fix registry server reference in image-pull-secret.yaml comment

Rationale:
- Backend server.js is written for Node.js/Express, not Bun.serve()
- Bun was causing CrashLoopBackOff due to incompatible server API
- Node.js provides better stability for production Express apps
- Fixed registry URLs to match actual Gitea service name in cluster

🤖 Generated with OpenCode

Co-Authored-By: OpenCode <noreply@opencode.com>

backend/Dockerfile

@@ -1,5 +1,5 @@
 # Multi-stage build for multi-architecture support (AMD64, ARM64)
-FROM --platform=$BUILDPLATFORM oven/bun:1-alpine AS builder
+FROM --platform=$BUILDPLATFORM node:20-alpine AS builder
 
 WORKDIR /app
 
@@ -7,13 +7,13 @@ WORKDIR /app
 COPY package*.json ./
 
 # Install dependencies
-RUN bun install --production
+RUN npm ci --production
 
 # Copy source code
 COPY . .
 
 # --- Production stage ---
-FROM --platform=$TARGETPLATFORM oven/bun:1-alpine
+FROM --platform=$TARGETPLATFORM node:20-alpine
 
 # Install curl for health checks and other utilities
 RUN apk add --no-cache curl wget
@@ -32,7 +32,7 @@ EXPOSE 5000
 
 # Health check
 HEALTHCHECK --interval=30s --timeout=3s --start-period=40s \
-  CMD bun -e "fetch('http://localhost:5000/api/health').then(r=>process.exit(r.ok?0:1))"
+  CMD curl -f http://localhost:5000/api/health || exit 1
 
 # Start server
-CMD ["bun", "server.js"]
+CMD ["node", "server.js"]