Add auth and session management

[LumePart]

This PR adds in-memory authentication and session management for the UI/backend

Thanks to @dammitjeff for the UI PR! (#116)

Included changes

• Authentication using credentials defined via environment variables
• Basic login page
• Ability to protect API endpoints behind authentication
• Session creation and validation
• CSRF token generation and verification
• UI API endpoints moved to /api/ui/
• Minor backend cleanup/refactoring

A larger backend refactor is planned in the future

Test

1. Clone the repository feat/backend branch:

git clone -b feat/backend https://github.com/LumePart/Explo.git explo-backend
cd explo-backend

2. Copy over your .env file (optional)
3. Open docker-compose.yaml, replace:
   image: ghcr.io/lumepart/explo:latest
   with
   build: .

Add under environment:

• UI_USERNAME=test
• UI_PASSWORD=testpass

4. run docker compose up -d --build
5. access the UI http://{YOUR_IP}:7288 and login with credentials defined in docker-compose

docker-compose sample:

services:
  explo:
    build: .
    restart: unless-stopped
    container_name: explo-backend
    ports:
      - "7288:7288"
    extra_hosts:
      - "host.docker.internal:host-gateway"
    volumes:
      - /path/to/.env:/opt/explo/.env
      - /data/music/explo:/data/ # has to be in the same path you have your music system pointed to (it's recommended to put explo under a subfolder)
      # - /path/to/slskd/downloads:/slskd/ # if using slskd and MIGRATE_DOWNLOADS is set to true in .env
      # - $PLAYLIST_DIR:$PLAYLIST_DIR # for MPD. Both paths should be as defined in .env (e.g /my/playlists/:/my/playlists/)

      # - /path/to/cookies.txt:/opt/explo/cookies.txt # Path to optional cookies file (for yt-dlp)
    environment:
      - UI_USERNAME=test
      - UI_PASSWORD=testpass
      - TZ=UTC 

PS! /api/ui/config is not behind authentication at the moment, due to it breaking the login page. (needs front end fix)
Feedback is welcome