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/store/blocks.go

@@ -0,0 +1,99 @@
+// Package store persists Kamado runtime data that must survive process
+// restarts. Currently: the found-block history. Uses modernc.org/sqlite
+// (pure Go, no CGO) so the kamado-api binary stays statically linkable.
+package store
+
+import (
+	"database/sql"
+	"errors"
+	"fmt"
+	"time"
+
+	_ "modernc.org/sqlite"
+)
+
+// BlockStore persists found blocks to a single SQLite file.
+type BlockStore struct {
+	db *sql.DB
+}
+
+// Block mirrors state.BlockRecord without the import cycle. The store
+// package is the lower layer; the state package converts to/from this
+// shape when reading and writing.
+type Block struct {
+	Height   int64
+	Hash     string
+	RewardBT float64
+	FoundAt  time.Time
+	Source   string
+}
+
+const schema = `
+CREATE TABLE IF NOT EXISTS blocks (
+    height     INTEGER PRIMARY KEY,
+    hash       TEXT NOT NULL DEFAULT '',
+    reward_btc REAL NOT NULL DEFAULT 0,
+    found_at   INTEGER NOT NULL,
+    source     TEXT NOT NULL DEFAULT ''
+);
+CREATE INDEX IF NOT EXISTS blocks_found_at_idx ON blocks(found_at);
+`
+
+// Open initializes the store at path, creating the schema if needed.
+// Callers are responsible for Close().
+func Open(path string) (*BlockStore, error) {
+	db, err := sql.Open("sqlite", path+"?_pragma=journal_mode(WAL)&_pragma=busy_timeout(5000)")
+	if err != nil {
+		return nil, fmt.Errorf("store: open %s: %w", path, err)
+	}
+	if _, err := db.Exec(schema); err != nil {
+		_ = db.Close()
+		return nil, fmt.Errorf("store: schema: %w", err)
+	}
+	return &BlockStore{db: db}, nil
+}
+
+func (s *BlockStore) Close() error {
+	return s.db.Close()
+}
+
+// InsertBlock is idempotent — duplicate heights are ignored so replayed
+// log events after a restart don't trip the primary key constraint.
+func (s *BlockStore) InsertBlock(b Block) error {
+	_, err := s.db.Exec(
+		`INSERT OR IGNORE INTO blocks(height, hash, reward_btc, found_at, source)
+		 VALUES (?, ?, ?, ?, ?)`,
+		b.Height, b.Hash, b.RewardBT, b.FoundAt.Unix(), b.Source,
+	)
+	return err
+}
+
+// Recent returns up to limit blocks, newest first.
+func (s *BlockStore) Recent(limit int) ([]Block, error) {
+	if limit <= 0 {
+		limit = 256
+	}
+	rows, err := s.db.Query(
+		`SELECT height, hash, reward_btc, found_at, source
+		 FROM blocks ORDER BY height DESC LIMIT ?`,
+		limit,
+	)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	out := make([]Block, 0, limit)
+	for rows.Next() {
+		var b Block
+		var unix int64
+		if err := rows.Scan(&b.Height, &b.Hash, &b.RewardBT, &unix, &b.Source); err != nil {
+			return nil, err
+		}
+		b.FoundAt = time.Unix(unix, 0).UTC()
+		out = append(out, b)
+	}
+	if err := rows.Err(); err != nil && !errors.Is(err, sql.ErrNoRows) {
+		return nil, err
+	}
+	return out, nil
+}