Switches from the docker/rust:1-bookworm container back to the rust-ci
runner label, and adopts sporah's build/test/audit job split with
cargo registry + sccache caching. Keeps feedsignal's workspace-specific
checks (native crates, web crate server/wasm-client features) and adds
clippy + cargo audit, which were previously missing.
Rust workspace with core (topic-affinity learning engine + relevance
scoring), db (sqlite/sqlx schema + repo), feeds (RSS/Atom fetch), llm
(rig + local Ollama embeddings/completion), and web (axum + Dioxus
fullstack UI, no separate JS stack). Two-stage relevance filtering
(embedding shortlist -> LLM judgment) and an engagement/surprise-based
topic affinity engine with daily decay. All crates compile and core's
affinity engine has passing unit tests; server and wasm client targets
of feedsignal-web both check clean.
