chore: gate and prepare v0.3.0 release

[yuyukosama2004]

R1 stage release gate

• bump backend, API, frontend, and lockfile versions to 0.3.0
• run a full Alembic base/head/check/downgrade/re-upgrade round trip in CI
• reject release tags that do not match every declared project version
• move CI and Release to GitHub-hosted Windows runners so pipelines do not depend on a local process
• update the public release and quality-check documentation

Local verification

• Ruff check and format: passed
• mypy (92 source files): passed
• Alembic round trip: passed
• pytest: 77 passed
• Prettier and ESLint: passed
• Vitest: 61 passed
• production build: passed
• matching v0.3.0 accepted and mismatched v0.2.0 rejected