feat: add platform registry with get_client() dispatch

app/services/platforms/__init__.py

@@ -0,0 +1,28 @@
+from __future__ import annotations
+
+from app.services.platforms.base import PostingStrategy, PostResult
+from app.services.platforms.reddit_post import RedditPostStrategy
+
+_REGISTRY: dict[str, PostingStrategy] = {
+    s.campaign_type: s()
+    for s in [
+        RedditPostStrategy,
+        # RedditCommentStrategy — added in Plan B
+        # BlogPostStrategy — added in Plan C
+    ]
+}
+
+SUPPORTED_PLATFORMS: frozenset[str] = frozenset(_REGISTRY)
+
+
+def get_client(campaign_type: str) -> PostingStrategy:
+    """Return the strategy instance for the given campaign type.
+
+    Raises ValueError for unknown types.
+    """
+    if campaign_type not in _REGISTRY:
+        raise ValueError(f"Unknown campaign type: {campaign_type!r}")
+    return _REGISTRY[campaign_type]
+
+
+__all__ = ["get_client", "SUPPORTED_PLATFORMS", "PostingStrategy", "PostResult"]

tests/services/platforms/test_registry.py

@@ -0,0 +1,17 @@
+import pytest
+from app.services.platforms import get_client, SUPPORTED_PLATFORMS
+from app.services.platforms.reddit_post import RedditPostStrategy
+
+
+def test_get_client_returns_reddit_post_strategy():
+    client = get_client("reddit_post")
+    assert isinstance(client, RedditPostStrategy)
+
+
+def test_get_client_unknown_type_raises():
+    with pytest.raises(ValueError, match="Unknown campaign type"):
+        get_client("nonexistent_type")
+
+
+def test_supported_platforms_contains_reddit_post():
+    assert "reddit_post" in SUPPORTED_PLATFORMS