Import upstream v0.16.25, stripped

Upstream commit: 3f657330c0f49a015a3a372fb59669b5cccbca6d
Enterprise-only files removed or emptied: 63
Enterprise-only snippets removed: 118 in 50 files
Dangling module declarations removed: 5
Edits turning enterprise off: 25
Third-party code: 14 files, 0 not in THIRD-PARTY.md
Renamed identifiers: 67 in 19 files
Verification: clean

The same Enterprise footprint as v0.16.24. The build check is clean
apart from the expected errors in the rebuilt-feature tests. A
bug-fix release: DKIM rotation, IMAP failed-login answers, DNSBL
multi-code scoring and negative TTLs, Pyzor on short messages, queue
quotas with an empty match, RocksDB info-log rotation, and
Autodiscover schema handling.

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,18 @@
+> [!IMPORTANT]
+> **If you are not an approved contributor, this pull request will be closed automatically.**
+>
+> Stalwart only accepts pull requests from approved contributors, and LLM-generated code is not accepted. Please read [CONTRIBUTING.md](https://github.com/stalwartlabs/stalwart/blob/main/CONTRIBUTING.md) before going further.
+>
+> **Discuss your change first at [support.stalw.art](https://support.stalw.art).** You can sign in with your GitHub account. If a maintainer approves the change, you will be added to the list of approved contributors and can open the pull request.
+>
+> **If this is a bug fix, also report the bug at [support.stalw.art](https://support.stalw.art).** Once a maintainer confirms it, an issue is created on your behalf.
+
+## What does this change and why?
+
+## Link to the support.stalw.art discussion
+
+## How was it tested?
+
+- [ ] I have read CONTRIBUTING.md.
+- [ ] I am an approved contributor and have discussed this change at [support.stalw.art](https://support.stalw.art).
+- [ ] Every line in this pull request was written or reviewed by me, and I can explain it.