Refresh README for FOSS professionalism and expanded audience #1353

Summary

Update the README to be more professional while keeping the project's tongue-in-cheek humour, and elevate Rackula's positioning as a FOSS project. This is the pre-release polish pass before the first CalVer-tagged release ships.

Motivation

The current README was written when Rackula was primarily a homelab tool. The project has since expanded to serve AV techs, IT students, and others. The README also buries self-hosting details (auth config, env vars) that belong in the Self-Hosting Guide, making the page too long and hard to scan.

Additionally, the CalVer cutover (#1802, #1315) and upcoming LXC deployment (#1211) require README updates to accurately represent versioning, badges, and new deployment options.

Changes

Structure & Tone

• Add descriptive subtitle + flavour tagline
• Add Features section with icon-prefixed bullets and a static screenshot (alongside existing GIF)
• Add "Who It's For" use case section
• Add contributing callout with good-first-issue link
• Move "Built With Claude" up as a differentiator
• Condense Acknowledgements to one-liner + link
• Tone: hybrid — professional structure with fun copy (keep personality, make it scannable)

Badges

• Keep License, GitHub Release, and Docker badges (work fine with CalVer)
• Add CalVer badge (signals versioning scheme at a glance)
• Replace coverage / test-ratio / test-time badges with a CI status badge (cleaner hero, less stale-looking)
• Preserve Star History

Deployment

• Add LXC deployment section — Proxmox community-scripts one-liner + manual install option
• Move deployment config details (auth env vars, CORS, cookie settings) to Self-Hosting Guide (link only in README)
• Keep Docker Run and Docker Compose sections

Links & Metadata

• Add GitHub Discussions link
• Add Changelog link (CHANGELOG.md)
• Move Documentation section higher
• Update copyright to 2025–2026 Gareth Evans

Versioning Accuracy

• Docker image badge references should work with CalVer tags (v26.6.0 etc.)
• Auth section: update local mode tracking reference (feat: Local authentication mode (username/password) #1117 status)

Design Document

See docs/plans/2026-02-27-readme-refresh-design.md (committed on branch).

Acceptance Criteria

• README follows hybrid tone (professional structure, fun copy)
• Features section with static screenshot added
• CalVer badge added
• Coverage badges replaced with CI status badge
• LXC deployment section added (community-scripts one-liner + manual option)
• Auth config details moved to Self-Hosting Guide (README links to docs)
• GitHub Discussions link added
• Changelog link added
• Copyright updated to 2025–2026
• Existing badges (License, Release, Docker) and Star History preserved
• All deployment config removed from README (linked to Self-Hosting Guide)