Fix Docker deployment issues - Redis connectivity & environment variables

[Mufeth7]

This PR resolves critical deployment issues in the Docker configuration that prevent note creation.

Problem

1. Redis Connection Failure: Missing REDIS_URL environment variable caused backend to fail connecting to Redis
2. Environment Variable Syntax: SIZE_LIMIT: 4 MiB (with space) would be parsed incorrectly by Rust's byte_unit
3. Port Conflicts: Frontend proxy (port 3000) tried reaching backend via port 8000, but Docker mapped port 80→8000 (Matches frontend proxy expectations from vite.config.js)

Impact

• ✅ Ensures Redis connectivity for note storage
• ✅ Prevents environment variable parsing errors
• ✅ Resolves frontend/backend port conflicts
• ✅ Explicit network definition improves deployment reliability