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/internal/state/aggregator.go

@@ -12,6 +12,7 @@ import (
 
 	"github.com/kamadopool/kamado-api/internal/bitcoind"
 	"github.com/kamadopool/kamado-api/internal/ckpool"
+	"github.com/kamadopool/kamado-api/internal/store"
 )
 
 // Snapshot is the merged view served to the UI. All fields are safe to
@@ -49,6 +50,7 @@ type Snapshot struct {
 type Aggregator struct {
 	CK       *ckpool.Client
 	RPC      *bitcoind.RPC
+	Store    *store.BlockStore // optional; nil disables persistence
 	Interval time.Duration
 	Log      *slog.Logger
 
@@ -78,8 +80,10 @@ func New(ck *ckpool.Client, rpc *bitcoind.RPC, interval time.Duration, log *slog
 
 // Run blocks until ctx is cancelled, refreshing the snapshot every Interval.
 // It runs one immediate refresh at startup so readers don't see an empty
-// snapshot after ctx launches the goroutine.
+// snapshot after ctx launches the goroutine. Persisted block history is
+// loaded from the store before the first refresh.
 func (a *Aggregator) Run(ctx context.Context) {
+	a.loadPersistedBlocks()
 	a.refresh(ctx)
 	t := time.NewTicker(a.Interval)
 	defer t.Stop()