feat(storage): add S3 backend support and advanced upload limits

- Introduce S3-compatible storage backend support using minio-go.
- Add configuration options for local storage limits, box limits, and rate limiting.
- Implement storage backend selection (local vs S3) for anonymous and registered users.
- Add an `/admin/storage` management interface.
- Update documentation and environment examples with the new configuration variables.

backend/libs/services/settings_test.go

@@ -146,6 +146,44 @@ func TestDailyUsageAndCleanup(t *testing.T) {
 	}
 }
 
+func TestEffectiveUserPolicyUsesOverridesAndInheritance(t *testing.T) {
+	settings := newTestSettingsService(t)
+	policy, err := settings.UploadPolicy()
+	if err != nil {
+		t.Fatalf("UploadPolicy returned error: %v", err)
+	}
+	policy.UserDailyUploadMB = 100
+	policy.DefaultUserStorageMB = 200
+	policy.UserMaxDays = 30
+	policy.UserDailyBoxes = 40
+	policy.UserActiveBoxes = 50
+	policy.UserStorageBackend = "local"
+
+	overrideDaily := 300.0
+	overrideQuota := 0.0
+	overrideDays := 12
+	overrideBackend := "bucket-1"
+	user := User{
+		ID: "user-1",
+		Policy: UserPolicy{
+			DailyUploadMB:    &overrideDaily,
+			StorageQuotaMB:   &overrideQuota,
+			MaxDays:          &overrideDays,
+			StorageBackendID: &overrideBackend,
+		},
+	}
+	effective := settings.EffectivePolicyForUser(policy, user)
+	if effective.DailyUploadMB != overrideDaily || effective.MaxDays != overrideDays || effective.StorageBackendID != overrideBackend {
+		t.Fatalf("effective policy did not use overrides: %+v", effective)
+	}
+	if effective.StorageQuotaSet {
+		t.Fatalf("zero storage quota override should mean unlimited: %+v", effective)
+	}
+	if effective.DailyBoxes != policy.UserDailyBoxes || effective.ActiveBoxes != policy.UserActiveBoxes {
+		t.Fatalf("effective policy did not inherit box caps: %+v", effective)
+	}
+}
+
 func newTestSettingsService(t *testing.T) *SettingsService {
 	t.Helper()
 	root := t.TempDir()