feat(podcasts): constrain monologue briefs to a single speaker

surfsense_backend/app/podcasts/schemas/spec.py

@@ -148,6 +148,14 @@ class PodcastSpec(BaseModel):
             raise ValueError("speaker slots must be unique")
         return self
 
+    @model_validator(mode="after")
+    def _check_style_speakers(self) -> PodcastSpec:
+        # One voice is what "monologue" means; letting extra speakers through
+        # would force drafting to silently pick a winner.
+        if self.style is PodcastStyle.MONOLOGUE and len(self.speakers) != 1:
+            raise ValueError("a monologue has exactly one speaker")
+        return self
+
     def speaker_for(self, slot: int) -> SpeakerSpec:
         """Return the speaker bound to ``slot`` or raise if none matches."""
         for speaker in self.speakers:

surfsense_backend/tests/unit/podcasts/test_spec.py

@@ -14,6 +14,7 @@ from pydantic import ValidationError
 from app.podcasts.schemas import (
     DurationTarget,
     PodcastSpec,
+    PodcastStyle,
     SpeakerRole,
     SpeakerSpec,
     Transcript,
@@ -80,6 +81,27 @@ def test_a_brief_needs_at_least_one_speaker():
         )
 
 
+def test_a_monologue_brief_carries_exactly_one_speaker():
+    spec = PodcastSpec(
+        language="en",
+        style=PodcastStyle.MONOLOGUE,
+        speakers=[_speaker(0)],
+        duration=DurationTarget(min_minutes=5, max_minutes=10),
+    )
+    assert spec.style is PodcastStyle.MONOLOGUE
+
+
+def test_a_monologue_brief_rejects_multiple_speakers():
+    """One voice is what 'monologue' means; a second speaker is a user error."""
+    with pytest.raises(ValidationError):
+        PodcastSpec(
+            language="en",
+            style=PodcastStyle.MONOLOGUE,
+            speakers=[_speaker(0), _speaker(1, voice_id="kokoro:af_bella")],
+            duration=DurationTarget(min_minutes=5, max_minutes=10),
+        )
+
+
 def test_duration_rejects_an_inverted_range():
     """A max below the min is a user error caught at the brief gate."""
     with pytest.raises(ValidationError):