Persist found blocks in SQLite (Phase 2b.5)

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.

api/go.mod

@@ -1,3 +1,17 @@
 module github.com/kamadopool/kamado-api
 
 go 1.22
+
+require modernc.org/sqlite v1.34.5
+
+require (
+	github.com/dustin/go-humanize v1.0.1 // indirect
+	github.com/google/uuid v1.6.0 // indirect
+	github.com/mattn/go-isatty v0.0.20 // indirect
+	github.com/ncruces/go-strftime v0.1.9 // indirect
+	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
+	golang.org/x/sys v0.22.0 // indirect
+	modernc.org/libc v1.55.3 // indirect
+	modernc.org/mathutil v1.6.0 // indirect
+	modernc.org/memory v1.8.0 // indirect
+)