Add Jinja2 configuration templates to 4.0.0 changelog

Author: vstakhov

changelogs/4.0.0.md

@@ -19,6 +19,7 @@ title: "Major Release with checkv3 Protocol, Built-in Fasttext, Multi-Flag Fuzzy
 
 ## Added
 
+- **Jinja2 configuration templates**: Configuration files are now preprocessed by the [Lupa Jinja2-compatible template engine](/configuration/templates) before UCL parsing. Environment variables prefixed with `RSPAMD_` are exposed as the `env` table in templates; modified delimiters (`{= =}` for expressions, `{% %}` for control structures) avoid conflicts with UCL syntax. New validation filters (`mandatory`, `require_int`, `require_number`, `require_bool`, `require_duration`, `require_json`, `fromjson`) abort startup with a clear error on invalid input, enabling container-ready configuration validation without shell entrypoint scripts. ([#5938](https://github.com/rspamd/rspamd/pull/5938), [#5941](https://github.com/rspamd/rspamd/pull/5941))
 - **checkv3 multipart protocol**: New `/checkv3` endpoint using `multipart/form-data` requests and `multipart/mixed` responses; metadata sent as structured JSON/msgpack instead of HTTP headers, per-part zstd compression, optional body part for rewritten messages, and zero-copy piecewise writev for responses. Use `rspamc --protocol-v3` or `rspamc --msgpack` to activate. ([#5880](https://github.com/rspamd/rspamd/pull/5880))
 - **Pluggable Hyperscan cache backend**: Hyperscan compilation and caching moved to an async Lua backend with Redis-based shared database support across workers and hosts. Async compilation prevents blocking the main event loop; self-healing cache auto-detects stale blobs and triggers recompile; small databases compiled in-memory without file caching. ([#5813](https://github.com/rspamd/rspamd/pull/5813), [#5952](https://github.com/rspamd/rspamd/pull/5952))
 - **Multi-flag fuzzy hashes**: A single fuzzy hash can now carry up to 8 flags simultaneously, allowing multiple rules to match the same digest with independent flag/value pairs. Redis update path rewritten in Lua with EVALSHA and NOSCRIPT recovery. Backward-compatible epoch 12 wire protocol with highest-value flag promoted to the primary slot. Fuzzy hashes now stored in Redis history. ([#5894](https://github.com/rspamd/rspamd/pull/5894), [#5860](https://github.com/rspamd/rspamd/pull/5860))