New internal/store package wraps modernc.org/sqlite (pure Go,
no CGO) with a BlockStore that exposes Open/Close/InsertBlock/
Recent. The aggregator now accepts an optional *store.BlockStore;
on Run() it loads up to maxBlockHistory rows from the store before
the first refresh, and each ingested block gets written to the
DB before being appended to the in-memory ring.
main.go opens the store at cfg.DBPath and logs a warning + falls
back to in-memory-only if the file can't be created — a broken
data volume shouldn't stop the pool from running.
InsertBlock uses INSERT OR IGNORE on the height primary key so
replayed log events after a restart are harmless.
