fix(config): harden default JWT secret

Replace known development JWT secret placeholders with an ephemeral runtime secret during config loading.

Return LoadInfo so startup can warn when token persistence depends on a stable configured secret.

Update config docs and tests for default, legacy placeholder, custom, env, and empty secret behavior.

internal/config/load_test.go

@@ -9,7 +9,7 @@ import (
 
 func TestDefaults(t *testing.T) {
 	v := New()
-	cfg, err := Load(v, "")
+	cfg, info, err := Load(v, "")
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -36,6 +36,16 @@ func TestDefaults(t *testing.T) {
 			}
 		})
 	}
+
+	if !info.EphemeralJWTSecret {
+		t.Fatal("expected default jwt.secret to be replaced with an ephemeral secret")
+	}
+	if cfg.JWT.Secret == DefaultJWTSecret {
+		t.Fatal("default jwt.secret was not replaced")
+	}
+	if cfg.JWT.Secret == "" {
+		t.Fatal("ephemeral jwt.secret is empty")
+	}
 }
 
 func TestFromYAML(t *testing.T) {
@@ -67,10 +77,13 @@ jwt:
 	}
 
 	v := New()
-	cfg, err := Load(v, path)
+	cfg, info, err := Load(v, path)
 	if err != nil {
 		t.Fatal(err)
 	}
+	if info.EphemeralJWTSecret {
+		t.Fatal("custom jwt.secret should not be replaced")
+	}
 
 	if cfg.Server.Host != "127.0.0.1" {
 		t.Errorf("server.host = %q, want %q", cfg.Server.Host, "127.0.0.1")
@@ -102,10 +115,13 @@ func TestEnvOverride(t *testing.T) {
 	t.Setenv("MYGO_DATABASE_SQLITE_PATH", "/env/path/db.sqlite")
 
 	v := New()
-	cfg, err := Load(v, "")
+	cfg, info, err := Load(v, "")
 	if err != nil {
 		t.Fatal(err)
 	}
+	if info.EphemeralJWTSecret {
+		t.Fatal("env jwt.secret should not be replaced")
+	}
 
 	if cfg.Server.Port != 8080 {
 		t.Errorf("server.port = %d, want %d", cfg.Server.Port, 8080)
@@ -137,7 +153,7 @@ server:
 	t.Setenv("MYGO_SERVER_PORT", "9999")
 
 	v := New()
-	cfg, err := Load(v, path)
+	cfg, _, err := Load(v, path)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -164,7 +180,7 @@ server:
 	}
 
 	v := New()
-	_, err := Load(v, path)
+	_, _, err := Load(v, path)
 	if err == nil {
 		t.Fatal("expected error for port 0, got nil")
 	}
@@ -183,7 +199,7 @@ jwt:
 	}
 
 	v := New()
-	_, err := Load(v, path)
+	_, _, err := Load(v, path)
 	if err == nil {
 		t.Fatal("expected error for empty jwt.secret, got nil")
 	}
@@ -191,12 +207,48 @@ jwt:
 
 func TestExplicitConfigFileNotFound(t *testing.T) {
 	v := New()
-	_, err := Load(v, "/nonexistent/path/config.yaml")
+	_, _, err := Load(v, "/nonexistent/path/config.yaml")
 	if err == nil {
 		t.Fatal("expected error when explicitly specifying a nonexistent config file")
 	}
 }
 
+func TestWeakJWTSecretsAreReplaced(t *testing.T) {
+	tests := []string{
+		DefaultJWTSecret,
+		"change-me-in-production",
+		"changeme-in-production",
+	}
+
+	for _, secret := range tests {
+		t.Run(secret, func(t *testing.T) {
+			dir := t.TempDir()
+			path := filepath.Join(dir, "config.yaml")
+
+			yaml := "jwt:\n    secret: " + secret + "\n"
+			if err := os.WriteFile(path, []byte(yaml), 0644); err != nil {
+				t.Fatal(err)
+			}
+
+			v := New()
+			cfg, info, err := Load(v, path)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			if !info.EphemeralJWTSecret {
+				t.Fatal("expected weak jwt.secret to be replaced")
+			}
+			if cfg.JWT.Secret == secret {
+				t.Fatal("weak jwt.secret was not replaced")
+			}
+			if cfg.JWT.Secret == "" {
+				t.Fatal("ephemeral jwt.secret is empty")
+			}
+		})
+	}
+}
+
 func TestJWTConfigAccessDuration(t *testing.T) {
 	j := JWTConfig{AccessTTL: 15 * time.Minute}
 	if j.AccessTTL != 15*time.Minute {