storage: add speedtests table + Save/List/Delete + retention

Adds a Store-method API for the RecordSpeedTest RPC (Task 8) and the
retention sweep (Task 14):
  - SaveSpeedTest / ListRecentSpeedTests / DeleteSpeedTestsBefore
  - SpeedTest struct (no json tags; that decision deferred to Task 10)

Schema lives in migrations/002_speedtests.sql to match the existing
embed.FS pattern (rather than inline in Go). The migrate() loop now
iterates every *.sql in lexical order, so future migrations drop in
without code changes. Both files use CREATE ... IF NOT EXISTS so the
existing 001 migration remains idempotent on already-initialised DBs.

CHECK constraints enforce node IN ('fl','pl1') and non-negative metric
values at the DB level. country has no DB CHECK — validation belongs
to the gRPC handler in Task 8.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/storage/speedtests.go

@@ -0,0 +1,65 @@
+package storage
+
+import (
+	"context"
+	"fmt"
+)
+
+// SpeedTest is a single client-side speedtest result reported via the
+// RecordSpeedTest RPC. Timestamp is unix seconds. Node is 'fl' or 'pl1'
+// (DB-level CHECK). Country is ISO-3166-1 alpha-2 (validated at the
+// gRPC handler, not at the DB).
+type SpeedTest struct {
+	ID        int64
+	Timestamp int64
+	Node      string
+	Country   string
+	DlMbps    float64
+	UlMbps    float64
+	PingMs    float64
+	JitterMs  float64
+}
+
+// SaveSpeedTest inserts a speedtest row and returns the new id.
+func (s *Store) SaveSpeedTest(ctx context.Context, st SpeedTest) (int64, error) {
+	res, err := s.db.ExecContext(ctx, `
+		INSERT INTO speedtests(timestamp, node, country, dl_mbps, ul_mbps, ping_ms, jitter_ms)
+		VALUES(?, ?, ?, ?, ?, ?, ?)`,
+		st.Timestamp, st.Node, st.Country, st.DlMbps, st.UlMbps, st.PingMs, st.JitterMs)
+	if err != nil {
+		return 0, fmt.Errorf("insert speedtest: %w", err)
+	}
+	return res.LastInsertId()
+}
+
+// ListRecentSpeedTests returns up to limit speedtests ordered by id DESC
+// (newest first).
+func (s *Store) ListRecentSpeedTests(ctx context.Context, limit int) ([]SpeedTest, error) {
+	rows, err := s.db.QueryContext(ctx, `
+		SELECT id, timestamp, node, country, dl_mbps, ul_mbps, ping_ms, jitter_ms
+		FROM speedtests ORDER BY id DESC LIMIT ?`, limit)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	out := []SpeedTest{}
+	for rows.Next() {
+		var st SpeedTest
+		if err := rows.Scan(&st.ID, &st.Timestamp, &st.Node, &st.Country,
+			&st.DlMbps, &st.UlMbps, &st.PingMs, &st.JitterMs); err != nil {
+			return nil, err
+		}
+		out = append(out, st)
+	}
+	return out, rows.Err()
+}
+
+// DeleteSpeedTestsBefore deletes rows with timestamp < ts and returns the
+// number of rows removed. Used by the retention sweep (Task 14).
+func (s *Store) DeleteSpeedTestsBefore(ctx context.Context, ts int64) (int64, error) {
+	res, err := s.db.ExecContext(ctx, `DELETE FROM speedtests WHERE timestamp < ?`, ts)
+	if err != nil {
+		return 0, err
+	}
+	return res.RowsAffected()
+}