Bundle the spam filter rules with the server

The server fetched upstream's latest published rules from GitHub at run
time: a version nobody here tested, code-like expressions from an account
we don't control, and the upstream name as a default in the admin form.

The published rules of spam-filter v3.0.2 are now embedded
(resources/spam-filter/, MIT, in THIRD-PARTY.md) and used whenever no other
source is configured. An empty setting and upstream's old default both mean
the bundled rules, so existing installs switch without a settings change;
the URL stays an operator override (https:// or file://). The schema default
is dropped and its description says what empty means, and the strip's
rename pass does the same to each import.

Rules load on first boot as before, and again whenever the bundled version
differs from the last one loaded, which only adds missing rules and tags.
That brings the AI classifier's LLM_* scores to installs that predate them:
production has none today.

upstream-watch now also opens an issue when spam-filter publishes a newer
release; resources/spam-filter/README.md says how to take it.

The antispam test now runs on the bundled rules, the path production
takes; SPAM_RULES_URL tests another set. Unit tests cover the URL handling
and that the bundled rules parse and score the AI tags as the AI spec says.

resources/spam-filter/README.md

@@ -0,0 +1,32 @@
+# Bundled spam filter rules
+
+`spam-filter-rules.json.gz` is the published rules file of
+[spam-filter](https://github.com/stalwartlabs/spam-filter) **v3.0.2**,
+unmodified. The server embeds it (`crates/common/src/manager/spam_rules.rs`)
+and loads it whenever no other rules source is configured, so a release
+scores mail with the rules it was tested with, offline and with nothing to
+fetch. The rules URL setting stays an operator override.
+
+The rules are dual-licensed MIT or Apache-2.0, Copyright (C) 2024, Stalwart
+Labs LLC; the fork takes them under MIT, with the notice in `THIRD-PARTY.md`.
+
+They include the scores for the AI classifier's tags (`LLM_*`, 3.0 for the
+high-confidence spam categories, −3.0 for legitimate), which match
+`docs/spec/features/ai-spam-classification.md`.
+
+## Updating
+
+The `upstream-watch` workflow opens an issue when spam-filter publishes a
+newer release. To take it:
+
+1. Download `spam-filter-rules.json.gz` from that release, pinned by tag
+   (`releases/download/vX.Y.Z/…`, not `latest`), over this file.
+2. Set `BUNDLED_SPAM_RULES_VERSION` in `spam_rules.rs` and the version in
+   this README and in `THIRD-PARTY.md`.
+3. Run the antispam test (`STORE=RocksDb RUST_MIN_STACK=16777216 cargo test
+   -p tests --lib -- smtp::inbound::antispam::antispam --exact`) and fix
+   expectations the new rules change, knowingly.
+
+On the next start each server loads the new version once. Loading only adds
+rules and tags that are missing; it never changes an existing one, so an
+operator's own adjustments survive.