fetchImage needs AbortSignal.timeout, bound picked from Pluto's own latency data

Gap 2 of PLUTO-691 (MARS-343 port). image-api.ts:80 fetchImage is a bare fetch, no AbortSignal/timeout — a stalled upstream to api.pensanta.com runs until the platform kills the function. Do NOT inherit Mars's 20s: Pluto's proxy sits behind auth+ownership and serves avatars on page render, tolerable stall is likely shorter. Needs db-pluto-cc's p50/p95/p99 on the image-fetch path before picking a number. Coder cannot self-serve this (lane gate on latency-distribution query).