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/sqlite.go

@@ -5,6 +5,7 @@ import (
 	"database/sql"
 	"embed"
 	"fmt"
+	"sort"
 	"time"
 
 	"github.com/bergabruh/stats-gateway/internal/probes"
@@ -42,13 +43,32 @@ func Open(path string) (*Store, error) {
 // Close closes the underlying database.
 func (s *Store) Close() error { return s.db.Close() }
 
+// migrate applies every SQL file under migrations/ in lexical filename order.
+// Each file is executed wholesale (statements are CREATE ... IF NOT EXISTS,
+// so re-applying on an existing DB is a no-op).
 func (s *Store) migrate() error {
-	body, err := migrations.ReadFile("migrations/001_init.sql")
+	entries, err := migrations.ReadDir("migrations")
 	if err != nil {
 		return err
 	}
-	_, err = s.db.Exec(string(body))
-	return err
+	names := make([]string, 0, len(entries))
+	for _, e := range entries {
+		if e.IsDir() {
+			continue
+		}
+		names = append(names, e.Name())
+	}
+	sort.Strings(names)
+	for _, name := range names {
+		body, err := migrations.ReadFile("migrations/" + name)
+		if err != nil {
+			return fmt.Errorf("read migration %s: %w", name, err)
+		}
+		if _, err := s.db.Exec(string(body)); err != nil {
+			return fmt.Errorf("apply migration %s: %w", name, err)
+		}
+	}
+	return nil
 }
 
 // InsertSample stores a probe result for the given node.