T57: Unified indexer query planner

[thedancingdeveloper]

One cache, one rate limiter, one query plan across all media types. Today Sonarr and Radarr independently hammer the same tracker with separate limiters — fewer requests here means materially lower ban risk.

Acceptance: measured reduction in duplicate indexer requests.

[thedancingdeveloper]

🤖 Vogt validation pass — 2026-08-20 (automated review vs dd26a0aa)

Valid and still pending. crates/stackarr-indexer/src/search.rs fans out per-indexer searches and only deduplicates the returned results (deduplicate()); it holds no shared query plan. A grep across the indexer crate finds no RateLimiter/Semaphore/governor rate limiter, no request cache, and no QueryPlanner, so there is nothing yet that collapses duplicate outbound indexer requests across media types — the 'measured reduction in duplicate indexer requests' acceptance is unmet.