feat: scaffold FastAPI backend with ports-and-adapters layout

backend/app/ports/protocols.py

@@ -0,0 +1,32 @@
+"""Port definitions — every adapter implements one of these protocols."""
+
+from collections.abc import AsyncIterator
+from typing import Protocol
+
+from app.domain.models import Recommendation, TasteProfile, Track
+
+
+class MusicCatalog(Protocol):
+    """Resolve, verify, and personalise against a music catalog."""
+
+    async def search_track(self, title: str, artist: str) -> Track | None: ...
+
+    async def fetch_taste_profile(self) -> TasteProfile: ...
+
+
+class Recommender(Protocol):
+    """The two LLM calls: intent + candidates, then streamed rerank."""
+
+    async def propose_candidates(
+        self, query: str, profile_summary: str
+    ) -> list[tuple[str, str]]: ...
+
+    def rerank(
+        self, query: str, profile_summary: str, grounded: list[Track]
+    ) -> AsyncIterator[Recommendation]: ...
+
+
+class PlaylistWriter(Protocol):
+    """Persist a recommendation set as a playlist."""
+
+    async def create_playlist(self, name: str, tracks: list[Track]) -> str: ...