perf(playout): parallelize playlist validation with dedup and worker pool

CHANGELOG.md

@@ -17,6 +17,7 @@
 - **R2 retention by run count + size**: Playout now keeps the newest `R2_KEEP_RUNS` run folders (default 2) and enforces an `R2_MAX_RETENTION_BYTES` cap (default 10 GiB), replacing the old 24h time-based `R2_SEGMENT_RETENTION_HOURS` cleanup. Cleanup runs after each new run plus an hourly backstop, and never deletes the live manifest. Adds `linearvoid_playout_r2_runs_retained` and `linearvoid_playout_r2_bytes_retained` gauges.
 - **Cloudflare Workers frontend hosting**: Replaced Cloudflare Pages with a Cloudflare Worker (`linearvoid-frontend`) that serves the static SPA via Workers Static Assets. Added `frontend/wrangler.toml` and renamed the deploy workflow to `.gitea/workflows/workers-deploy.yml` (builds the SPA and runs `npx wrangler deploy`). Architecture, setup/deployment, and ops documentation updated from Pages to Workers.
 - **Viewer session cookie cross-site**: The `linearvoid_user_session` cookie now uses `SameSite=None; Secure` over HTTPS (so it is sent cross-site from the Workers-served SPA) and `SameSite=Lax` over plain HTTP (local dev). The admin session cookie remains `SameSite=Strict`.
+- **Parallel playout startup validation**: `ValidatePlaylist` now deduplicates repeated paths and probes unique files concurrently through a bounded worker pool (`PLAYOUT_VALIDATE_WORKERS`, default `runtime.NumCPU()`); `ValidateVideo` runs its stream/format and keyframe `ffprobe` probes concurrently; and `main()` performs the strict validation once (removing the redundant `-validate` pre-step in `start-playout.sh`). This sharply cuts startup time on large or duplicated playlists.
 
 ### Fixed
 
@@ -28,6 +29,7 @@
 - Handlers: `TestHandleAdminChatLog_ExcludesSystem`.
 - Frontend: `presence flash` test.
 - Playout: `TestCleanupOldRuns`, `TestCleanupOldRuns_SizeCap`, `TestR2KeepRuns`, `TestR2MaxRetentionBytes`, `TestR2RunFromKey`.
+- Playout: `TestValidateWorkersConfig`, `TestValidatePlaylist_Dedup`.
 
 ## [1.0.13] - 2026-08-13