rate-limiting: custom user-id RateIssuer for write endpoints

[disconsented]

Add a custom RateIssuer that keys authenticated requests by user id instead of IP, for the write/vote (and optionally login) buckets.

Scope

• Implement a RateIssuer that reads the authenticated user from the Depot (populated by validate_biscuit_token; see auth::get_user_from_depot, used at src/web/item.rs:370).
• Fall back to IP keying when no user is present.
• Wire it into the write/vote buckets so vote-stuffing and property spam are limited per account, not per shared NAT/proxy IP.

Depends on

• rate-limiting: per-route limit buckets (search/item/write/login) #54 — per-route buckets

Parent: #50