docs: adopt the no-CLA plus DCO policy and fix copyright terms

Brings the server in line with the contribution and copyright policy
already adopted in pgsty/mc.

Copyright terms were derived from the clock: startupBanner overwrote the
ldflags-injected CopyrightYear with time.Now().Year() and printed it as
the end of MinIO, Inc.'s term, so every January would have extended the
upstream copyright claim past the 2025 end of upstream development, and
release builds silently discarded the injected year. Both banners now
credit MinIO, Inc. for 2015-2025 and PGSTY from 2025 through the
release-stamped year, falling back to the current year in source builds.
NOTICE names PGSTY as the holder of the fork's modifications, matching
the banners and the packaging vendor field.

Contributions are accepted inbound=outbound under AGPL-3.0-or-later with
no CLA - the core is Copyright MinIO, Inc., so the combined work can
never be relicensed and a CLA would buy nothing. What a fork carrying a
downstream delta does need is provenance, so DCO 1.1 sign-off is now
mandatory and enforced by a workflow that rejects unsigned non-bot
commits. CONTRIBUTING documents sign-off, repair, cherry-pick provenance,
dual copyright headers, trailer preservation across squash merges, and
that assistive-tooling trailers carry no authorship or copyright claim.

verify-rebrand.sh pins the copyright split and the policy files so
neither can regress.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Feng Ruohang <rh@vonng.com>

README_ZH.md

@@ -221,6 +221,8 @@ go build -o silo .
 
 Issue 与 Pull Request 应说明受影响版本、复现步骤、影响、预期行为、测试与兼容性说明。大型改动请先提交 Issue 讨论。
 
+本项目不要求签署 CLA：贡献按项目许可证（AGPL-3.0-or-later，inbound=outbound）接收，贡献者保留自己的版权。每个提交都必须按照 [DCO](https://developercertificate.org/) 签署（`git commit -s`），详见 [`CONTRIBUTING.md`](CONTRIBUTING.md)。
+
 ## 背景
 
 本项目源于上游社区发行与维护模式的变化。维护者对相关变化的分析、替代方案评估与早期维护记录见以下文章：