docs: make the project ready for outside contributors

[TheCodeHeist-Coder]

Rexial was public but not actually contributable: there was no license, and the setup instructions pointed at a repository name that no longer exists, so a new contributor failed at the clone step.

Licensing

• Add MIT LICENSE. Without it the repository was implicitly all-rights-reserved, so nobody could legally fork or contribute.

Fix the stale Qtrive naming

• setup.md cloned TheCodeHeist-Coder/Qtrive, which does not resolve
• Navbar linked to the same dead repository
• Rename the package and dev container names to Rexial

Deliberately left as Qtrive: POSTGRES_DB, DATABASE_URL and the docker volume names (renaming orphans existing local dev databases), and the 'Qtrive-auth' localStorage key (renaming logs out every current user). These are runtime state, not branding.

Documentation

• Rewrite README: the previous one ended mid-code-block with an unclosed fence and had no license or contributing section
• Point the demo link at demo.mp4 rather than the space-containing filename
• Rename contribution.md to CONTRIBUTING.md so GitHub links it automatically on every new issue and pull request
• Fix the placeholder clone URLs inside it
• Add ROADMAP.md, CODE_OF_CONDUCT.md and SECURITY.md

Issue and PR templates

• Add bug report and feature request forms with required fields, so reports arrive with reproduction steps instead of one-line summaries
• config.yml links to good first issues, setup and architecture docs
• Add a pull request template

Verified: pnpm lint (19 pre-existing frontend errors, unchanged), check-types and build all pass; frontend tsc --noEmit is clean.