fix: pgtest left an anonymous volume behind on every run

The postgres image declares a VOLUME, so tearing the container down with
`docker rm -f` (no -v) orphaned its anonymous data volume each time the
test binary ran. Add -v to all three teardown sites, and state the
cleanup expectation in AGENTS.md so a killed run gets swept by hand.

backend/internal/pgtest/pgtest.go

@@ -40,7 +40,9 @@ func Main(m *testing.M) int {
 		fmt.Println("pgtest:", err)
 		return 1
 	}
-	defer exec.Command("docker", "rm", "-f", id).Run()
+	// -v: the postgres image declares a VOLUME, so an explicit rm without it
+	// leaves the anonymous data volume behind on every test run.
+	defer exec.Command("docker", "rm", "-f", "-v", id).Run()
 
 	adminURL = url
 	return m.Run()
@@ -85,7 +87,7 @@ func start() (id, url string, err error) {
 
 	port, err := exec.Command("docker", "port", id, "5432/tcp").Output()
 	if err != nil {
-		exec.Command("docker", "rm", "-f", id).Run()
+		exec.Command("docker", "rm", "-f", "-v", id).Run()
 		return "", "", fmt.Errorf("docker port: %w", err)
 	}
 	// "0.0.0.0:32768" (and possibly a second, IPv6 line); the port is all we want.
@@ -94,7 +96,7 @@ func start() (id, url string, err error) {
 		first[strings.LastIndex(first, ":")+1:])
 
 	if err := waitReady(url); err != nil {
-		exec.Command("docker", "rm", "-f", id).Run()
+		exec.Command("docker", "rm", "-f", "-v", id).Run()
 		return "", "", err
 	}
 	return id, url, nil