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/blocks.go

@@ -5,6 +5,7 @@ import (
 	"time"
 
 	"github.com/kamadopool/kamado-api/internal/logmon"
+	"github.com/kamadopool/kamado-api/internal/store"
 )
 
 // BlockRecord is a found block, merged from a logmon event with bitcoind
@@ -44,6 +45,17 @@ func (a *Aggregator) IngestBlockEvents(ctx context.Context, events <-chan logmon
 				}
 				cancel()
 			}
+			if a.Store != nil {
+				if err := a.Store.InsertBlock(store.Block{
+					Height:   rec.Height,
+					Hash:     rec.Hash,
+					RewardBT: rec.RewardBT,
+					FoundAt:  rec.FoundAt,
+					Source:   rec.Source,
+				}); err != nil {
+					a.Log.Warn("block persist failed", "height", rec.Height, "err", err)
+				}
+			}
 			pushed := a.appendBlock(rec)
 			a.Log.Info("block recorded", "height", rec.Height, "hash", rec.Hash)
 			// Push immediately so WebSocket clients see the solve
@@ -77,6 +89,35 @@ func (a *Aggregator) appendBlock(rec BlockRecord) Snapshot {
 	return snap
 }
 
+// loadPersistedBlocks seeds a.blocks from the store so history survives
+// restarts. Safe to call with a nil Store — becomes a no-op.
+func (a *Aggregator) loadPersistedBlocks() {
+	if a.Store == nil {
+		return
+	}
+	rows, err := a.Store.Recent(maxBlockHistory)
+	if err != nil {
+		a.Log.Warn("block history load failed", "err", err)
+		return
+	}
+	// Store returns newest-first; a.blocks is newest-last.
+	recs := make([]BlockRecord, 0, len(rows))
+	for i := len(rows) - 1; i >= 0; i-- {
+		r := rows[i]
+		recs = append(recs, BlockRecord{
+			Height:   r.Height,
+			Hash:     r.Hash,
+			RewardBT: r.RewardBT,
+			FoundAt:  r.FoundAt,
+			Source:   r.Source,
+		})
+	}
+	a.mu.Lock()
+	a.blocks = recs
+	a.mu.Unlock()
+	a.Log.Info("block history loaded", "count", len(recs))
+}
+
 // Blocks returns a copy of the recent block history, newest last.
 func (a *Aggregator) Blocks() []BlockRecord {
 	a.mu.RLock()