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/base.py

@@ -0,0 +1,23 @@
+from abc import ABC, abstractmethod
+from dto.post import Post
+
+class BaseConnector(ABC):
+    # Each subclass declares these at the class level
+    source_name: str       # machine-readable: "reddit", "youtube"
+    display_name: str      # human-readable: "Reddit", "YouTube"
+    required_env: list[str] = []  # env vars needed to activate
+
+    @classmethod
+    def is_available(cls) -> bool:
+        """Returns True if all required env vars are set."""
+        import os
+        return all(os.getenv(var) for var in cls.required_env)
+
+    @abstractmethod
+    def get_new_posts_by_search(self, 
+                                search: str = None, 
+                                category: str = None, 
+                                post_limit: int = 10, 
+                                comment_limit: int = 10
+                                ) -> list[Post]:
+        ...