[Glense] Integrate services: gateway routing and inter-service communication

[andjixi]

Summary

Chat service stores usernames on messages
The Message model and DB schema now include a Username field (max 50 chars). When a message is created, the username is extracted from the JWT and stored directly, so there's no need to call the Account service per message.

Video Catalogue resolves uploader usernames
The Videos controller now calls the Account service to resolve usernames for all videos in a list response and for individual video fetches. If the Account service is unreachable, videos are still returned with uploaderUsername: null.

API Gateway proxies Video and Chat services
Two new proxy controllers (VideoProxyController, ChatProxyController) forward frontend requests to the downstream services.

Frontend routes all API calls through the gateway
videoApi.js and chatService.js now point to the gateway instead of directly to each service.

Channel page shows real profile data
ChannelCard and ChannelDetail replaced hardcoded demo content with real data fetched from the Account service.

Cleanup
Removed seed-test-users.sh (replaced by seed.sh), removed hardcoded demo data from constants.jsx, rewrote README.md and service READMEs in English with architecture diagrams and API tables.