[backlog] Add /v1/rerank endpoint (upstream #591)

[DeliciousBuding]

    ## Source
    - Upstream: cita-777/metapi #591
    - Matrix: `docs/analysis/original-gap-matrix.md`
    - Category: feature-protocol
    - Status: missing · Priority: P1 · Backlog: yes
    - Epic: Protocol
    - Parent: G3 #10

    ## Evidence (from matrix)
    `handler/proxy/router.go` `RegisterProxyRoutes` has chat/messages/completions/responses/models/embeddings/images/videos/search/files — no `rerank` route or handler.

    ## Acceptance criteria (draft — do not implement in G3)
    - [ ] Proxy exposes `/v1/rerank` compatible with original MetAPI contract

• Routing selects capable upstream channels for rerank

• Auth, logging, and error shape match other proxy routes

• e2e or unit coverage for happy path + unsupported channel

    ## Out of scope for this issue filing
    Docs-only backlog. No product code until a later implementation wave claims this issue.