chore(license): relicense from MIT to AGPL-3.0-or-later

[findias]

Why

Aligns with the broader anti-censorship FOSS ecosystem (Hiddify, Marzban, Outline, AmneziaVPN, sing-box are all AGPL-3.0). AGPL's network-use clause closes the SaaS-clone vector that MIT left fully open while keeping the project welcoming to community contributions.

This is the final piece of the org-wide AGPL-3.0 relicense:

• raven-dashboard v1.7.0 — Phase 1, done
• xray-stats-exporter v1.0.3 — Phase 1, done
• Raven-server-install — Phase 2, PR #40
• Raven-subscribe — Phase 2, this PR

What this means for you

• Self-host operators — no change. AGPL only triggers obligations when you redistribute or run a modified version as a network service. Personal/team deployments are unaffected.
• Forks running a public managed service — must publish their modified source to users of the service, per AGPL §13.
• Code at or before commit f944cef5afe32e4519825e630d4ab00cf2be8d27 remains available under the MIT License per its original release. From this PR forward, AGPL-3.0-or-later.

Consent and authorship

Past copyright holders consented to the relicense:

• findias (project lead, 254 commits, original author)
• @johanconrad — 3 commits, 115 surviving lines (all in .github/workflows/test.yml + .github/actions/setup-go-env/action.yml). Consent recorded at Relicensing AlchemyLink repos to AGPL-3.0-or-later — request your consent for past contributions Raven-server-install#38: "Yes, OK to relicense" on 2026-05-06.

dependabot[bot] commits are excluded as automated dependency bumps do not generate copyright.

Changes

• LICENSE replaced with canonical AGPL-3.0 text (sha256 0d96a4ff68ad...).
• README.md and README.ru.md — badge + License section updated.
• CONTRIBUTING.md gains a top-level "License and Developer Certificate of Origin" section with the DCO v1.1 sign-off requirement for future contributions; existing Quality Gate + Lint Policy + nolint Rules + Security Notes sections are preserved unchanged.

Test plan

• License text matches gnu.org/licenses/agpl-3.0.txt byte-for-byte (verified via sha256).
• No code changes — purely metadata + docs. Unit tests, vet, staticcheck etc. should be unaffected.
• DCO check passes (sign-off in commit body).
• CI test workflow passes.

Tagging

Suggest tagging v0.2.0 on merge — minor bump for license change (last release was v0.1.8). Cross-repo audit trail lands in the release notes.