docs: fix readme facts and license metadata

[lesnik512]

Summary

Fixes factual and style problems in the readme, plus a non-SPDX license string in pyproject.

Changes

• readme.md:29 linked https://github.com/modern-python/modern-python-template, which returns 404 (gh api repos/modern-python/modern-python-template). Sentence removed; .copier-answers.yml left untouched.
• readme.md:18 said "Production-ready dockerized async REST API on ..." while pyproject.toml:4 describes the repo as a template. The description now opens with the pyproject string, reworded into a sentence. Also spells "Litestar" (was "LiteStar").
• readme.md:24 said ruff and ty do linting and formatting; ty is a type checker. Now "Linting and formatting with ruff, type checking with ty".
• readme.md:36 linked LICENSE relatively; now absolute, matching the other org repos.
• Headings: "Key Features" is sentence case; "### Description" and "### After git clone run" became ## (the page had no ## before the first ###).
• pyproject.toml:10 license = "MIT License" is not an SPDX expression: packaging.licenses.canonicalize_license_expression("MIT License") raises, and validate-pyproject (with packaging>=24.2) rejects the file. 27 other org repos use "MIT". With "MIT" the file validates and uv lock --check passes.

Checklist

• Lint and format pass (ruff)
• Type check passes (ty)
• Tests pass and new behavior is covered
• Build succeeds (uv build) if packaging or build config changed
• Repo metadata stays consistent across the three surfaces (GitHub description, pyproject description, profile blurb) if this touches packaging

uv build fails the same way on main (setuptools flat-layout discovery, no build-system table), so it could not confirm the metadata; validate-pyproject was used instead. Tests ran against a throwaway postgres:14 container: 31 passed, 100% coverage.