rate-limiting: enable salvo rate-limiter feature + scaffold limit.rs

[disconsented]

Enable Salvo's rate-limiter feature and add a place for the limiter middleware to live.

Scope

• Add rate-limiter to the salvo feature list in the root Cargo.toml (currently: affix-state, cookie, logging, oapi, serve-static, size-limiter — no rate limiter).
• Create src/web/limit.rs and wire it into src/web/mod.rs.
• Establish the building blocks: a guard (SlidingGuard), an in-process store (MokaStore), and an IP issuer, so later issues can attach per-route limiters as hoops.

Notes

• Confirm exact type names against salvo 0.89.3.
• MokaStore is in-process and assumes a single instance (true today) — document that assumption.

Parent: #50