feat(03b): add BYO custom proxy provider + bounded crawler rotation-retry

Add CustomProxyProvider (single endpoint or rotating pool via Scrapling ProxyRotator), registered as 'custom' alongside anonymous_proxies and selectable via PROXY_PROVIDER. Adds is_pool_backed to the ProxyProvider ABC + a zero-arg package helper. The web crawler does a bounded one-per-tier is_proxy_error rotation-retry gated on is_pool_backed() (single-endpoint providers no-op). Config/.env.example gain CUSTOM_PROXY_URL(S). Zero-arg getter contract unchanged for all consumers. Documents the proprietary boundary test (generic proxy infra stays Apache-2). Tests: provider, registry, crawler rotation (16).

Co-authored-by: Cursor <cursoragent@cursor.com>

surfsense_backend/tests/unit/utils/proxy/test_registry.py

@@ -0,0 +1,36 @@
+"""Unit tests for proxy provider selection (Phase 3b).
+
+``PROXY_PROVIDER`` selects the single app-wide provider; ``custom`` is now
+registered alongside ``anonymous_proxies``, and unknown values still warn and
+fall back to the default.
+"""
+
+import pytest
+
+from app.config import Config
+from app.utils.proxy import registry
+from app.utils.proxy.providers.anonymous_proxies import AnonymousProxiesProvider
+from app.utils.proxy.providers.custom import CustomProxyProvider
+
+pytestmark = pytest.mark.unit
+
+
+@pytest.fixture(autouse=True)
+def _reset_active_provider(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Clear the process-wide provider cache so each test resolves fresh."""
+    monkeypatch.setattr(registry, "_active_provider", None)
+
+
+def test_resolves_custom(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.setattr(Config, "PROXY_PROVIDER", "custom")
+    assert isinstance(registry.get_active_provider(), CustomProxyProvider)
+
+
+def test_resolves_anonymous(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.setattr(Config, "PROXY_PROVIDER", "anonymous_proxies")
+    assert isinstance(registry.get_active_provider(), AnonymousProxiesProvider)
+
+
+def test_unknown_falls_back_to_default(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.setattr(Config, "PROXY_PROVIDER", "does_not_exist")
+    assert isinstance(registry.get_active_provider(), AnonymousProxiesProvider)