fix: keep startup fast regardless of history size - #18
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The one-off metric-bucket backfill ran on the startup path before the HTTP listener, so an upgraded install with 30 days of raw samples served no requests until it finished — over 25 minutes on a router-class device. It now runs in a background goroutine; chart handlers keep reading raw tables until each range's buckets exist, so the service answers within seconds of a restart. The retention loop's first pass waits for the backfill to finish in the same goroutine: pruning raw rows while the backfill consumes them would leave an unrecoverable hole in the middle of the history. The bucket window predicate also gained sargable text bounds (timestamp >= datetime(?, 'unixepoch')) in front of the epoch cast: the cast alone is not index-usable and forced a full-table scan per backfill chunk. The cast stays as the exact predicate; the text bounds only prune through the timestamp index, so row selection is unchanged.
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After the 0.3.34 upgrade the router's MikroMan took >25 min to serve anything: the one-off bucket backfill blocks startup, and its window predicate (a strftime/cast expression) is not index-sargable, so every day-chunk rescans the whole raw table.
Two changes:
Measured on a synthetic 518k-row month: chunk scan 153ms -> 88ms in RAM; the effect on flash-backed SQLite I/O is the real point.