feat: add recommendation domain foundations

backend/app/domain/profile.py

@@ -0,0 +1,28 @@
+"""Compress Spotify taste signals for prompts and known-track filtering."""
+
+from app.domain.models import CompressedTasteProfile, TasteProfile, Track
+
+
+def compress_taste_profile(profile: TasteProfile) -> CompressedTasteProfile:
+    """Build compact prompt text and the complete known track identifier set."""
+    sections = (
+        _line("Short-term top artists", profile.short_term_artists),
+        _line("Long-term top artists", profile.long_term_artists),
+        _line("Short-term top tracks", _track_labels(profile.short_term_tracks)),
+        _line("Long-term top tracks", _track_labels(profile.long_term_tracks)),
+        _line("Saved-track sample", _track_labels(profile.saved_tracks)),
+    )
+    known_tracks = (*profile.short_term_tracks, *profile.long_term_tracks, *profile.saved_tracks)
+    return CompressedTasteProfile(
+        text="\n".join(sections),
+        known_track_ids=frozenset(track.id for track in known_tracks),
+    )
+
+
+def _track_labels(tracks: tuple[Track, ...]) -> tuple[str, ...]:
+    return tuple(f"{track.title} by {', '.join(track.artists)}" for track in tracks)
+
+
+def _line(label: str, values: tuple[str, ...]) -> str:
+    rendered_values = "; ".join(values) if values else "none"
+    return f"{label}: {rendered_values}"