fix: P2 security and correctness issues

P2-10: Change ActivateEmail from GET to POST - token now passed in
request body instead of URL query parameter for better security

P2-11: Change ValidateResetToken from GET to POST - token now passed
in request body instead of URL query parameter to prevent log leakage

P2-12: Note - /uploads static exposure remains (requires architectural
decision about file serving)

P2-13: cursor.Encode() now checks and returns empty string on JSON
marshaling error instead of silently ignoring

P2-14: initDefaultData and ensurePermissions now properly check and
propagate errors from RolePermission creation, and createDefaultPermissions
aggregates errors instead of silently continuing

P2-15: NewJWT now returns (nil, error) on initialization failure
instead of a partially initialized object. All callers updated to handle
the error return.

Backend routes updated:
- POST /auth/activate-email (was GET /activate)
- POST /auth/password/validate (was GET /reset-password)

Frontend updated to match new API endpoints.

internal/e2e/e2e_test.go

@@ -67,7 +67,10 @@ func setupRealServer(t *testing.T) (*httptest.Server, func()) {
 		t.Fatalf("数据库迁移失败: %v", err)
 	}
 
-	jwtManager := auth.NewJWT("test-secret-key-for-e2e", 15*time.Minute, 7*24*time.Hour)
+	jwtManager, err := auth.NewJWT("test-secret-key-for-e2e", 15*time.Minute, 7*24*time.Hour)
+	if err != nil {
+		t.Fatalf("failed to create JWT manager: %v", err)
+	}
 	l1Cache := cache.NewL1Cache()
 	l2Cache := cache.NewRedisCache(false)
 	cacheManager := cache.NewCacheManager(l1Cache, l2Cache)