feat(playout): replace time-based R2 retention with run-count and size caps

CHANGELOG.md

@@ -14,12 +14,14 @@
 - **Reconnect grace period**: Disconnects schedule a delayed "left" announcement (5s); a reconnect within 5s cancels it and skips "joined", eliminating join/leave spam on brief reconnects.
 - **HLS segment duration 2s**: Default `HLS_SEGMENT_DURATION` changed from 6 to 2 and `HLS_WINDOW_SIZE` from 24 to 12 (2s × 12 = 24s live window), reducing end-to-end latency; the 2s GOP was already in place.
 - **Chat font sizing**: `.message-text` set to `0.95rem` (slightly smaller than the default, larger than the `0.83rem` system/flash text).
+- **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.
 
 ### Tests
 
 - WebSocket: `TestHub_Broadcast_SystemNotPersisted`, `TestHub_ReconnectGracePeriod`.
 - Handlers: `TestHandleAdminChatLog_ExcludesSystem`.
 - Frontend: `presence flash` test.
+- Playout: `TestCleanupOldRuns`, `TestCleanupOldRuns_SizeCap`, `TestR2KeepRuns`, `TestR2MaxRetentionBytes`, `TestR2RunFromKey`.
 
 ## [1.0.13] - 2026-08-13