feat(connectors): add base connector and registry for detection

Idea is to have a "plugin-type" system, where new connectors can extend the `BaseConnector` class and implement the fetch posts method.

These are automatically detected by the registry, and automatically used in new Flask endpoints that give a list of possible sources.

Allows for an open-ended system where new data scrapers / API consumers can be added dynamically.

server/connectors/registry.py

@@ -0,0 +1,25 @@
+import pkgutil
+import importlib
+import connectors
+from connectors.base import BaseConnector
+
+def _discover_connectors() -> list[type[BaseConnector]]:
+    """Walk the connectors package and collect all BaseConnector subclasses."""
+    for _, module_name, _ in pkgutil.iter_modules(connectors.__path__):
+        if module_name in ("base", "registry"):
+            continue
+        importlib.import_module(f"connectors.{module_name}")
+
+    return [
+        cls for cls in BaseConnector.__subclasses__()
+        if cls.source_name  # guard against abstract intermediaries
+    ]
+
+def get_available_connectors() -> list[type[BaseConnector]]:
+    return [c for c in _discover_connectors() if c.is_available()]
+
+def get_connector_metadata() -> list[dict]:
+    return [
+        {"id": c.source_name, "label": c.display_name}
+        for c in get_available_connectors()
+    ]