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/cmd/kamado-api/main.go

@@ -21,6 +21,7 @@ import (
 	"github.com/kamadopool/kamado-api/internal/httpapi"
 	"github.com/kamadopool/kamado-api/internal/logmon"
 	"github.com/kamadopool/kamado-api/internal/state"
+	"github.com/kamadopool/kamado-api/internal/store"
 )
 
 func main() {
@@ -41,7 +42,23 @@ func main() {
 
 	ck := ckpool.New(cfg.CKPoolSockDir)
 	rpc := bitcoind.NewRPC(cfg.BitcoinRPCURL, cfg.BitcoinRPCUser, cfg.BitcoinRPCPassword, cfg.BitcoinRPCTimeout)
+
+	// Persistent block history. Non-fatal if it can't be opened — the
+	// aggregator falls back to an in-memory ring so the pool keeps
+	// running even with a broken data volume.
+	var blockStore *store.BlockStore
+	if cfg.DBPath != "" {
+		if s, err := store.Open(cfg.DBPath); err != nil {
+			log.Warn("block store open failed, running without persistence", "path", cfg.DBPath, "err", err)
+		} else {
+			blockStore = s
+			defer blockStore.Close()
+			log.Info("block store opened", "path", cfg.DBPath)
+		}
+	}
+
 	agg := state.New(ck, rpc, cfg.PollInterval, log)
+	agg.Store = blockStore
 
 	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
 	defer stop()