feat: add offline deployment support

[NotYuSheng]

Summary

• Adds docker-compose.offline.yml that uses pre-built image names instead of build contexts, for air-gapped environments
• Adds scripts/pull-and-save-images.sh — pulls third-party images from Docker Hub, builds backend/nginx locally, and saves all as .tar files under images/
• Adds scripts/load-images.sh — loads all .tar files into Docker on the offline machine

Usage

# Internet-connected machine
bash scripts/pull-and-save-images.sh

# Transfer: images/, docker-compose.offline.yml, .env, scripts/load-images.sh

# Offline machine
bash scripts/load-images.sh
docker compose -f docker-compose.offline.yml up -d

Test plan

• Run pull-and-save-images.sh on a machine with internet access and verify all 5 .tar files are created in images/
• Run load-images.sh and confirm all images load without errors
• Start stack with docker compose -f docker-compose.offline.yml up -d and verify all services come up healthy

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request introduces comprehensive issue templates, updates the project documentation with new features and tech stack details, and adds support for offline deployments via new Docker Compose configurations and image management scripts. Feedback focuses on improving the robustness of the offline setup, including replacing curl with wget in healthchecks, defaulting LLM endpoints to local providers, pinning Docker image versions for determinism, and reducing redundancy in the image-saving scripts.