feat: Create and reuse existing session

[monstermuffin]

This fixes #87, qSticky currently creates a new qBittorrent session every poll interval, which causes repeated /api/v2/auth/login requests and noisy qBittorrent logs. This change keeps the existing Gluetun behavior but removes the unnecessary qB logins.

Reuse the qBittorrent aiohttp session between poll cycles instead of creating a new session and logging in on every check.

Changes

• Keep a persistent qBittorrent aiohttp.ClientSession for the lifetime of qSticky.
• Track qBittorrent auth state so login only happens when needed.
• Retry once by recreating the qB session after auth loss or connection failure.
• Route qB preference reads and port updates through the shared request flow.
• Update the Docker build workflow so the qbit-session branch is built for testing.

[monstermuffin]

Merged after testing.

[github-actions]

🎉 This PR is included in version 2.3.2 🎉

The release is available on GitHub release

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