feat(api): SessionRepository, session/song schemas with tags

Adds RehearsalSessionRepository (get_or_create, list_for_band with
counts, get_with_songs). Adds RehearsalSession schemas (Read, Detail,
Update). Extends SongRead/SongUpdate with tags and session_id fields.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

api/src/rehearsalhub/schemas/rehearsal_session.py

@@ -0,0 +1,27 @@
+import uuid
+from datetime import datetime
+
+from pydantic import BaseModel, ConfigDict
+
+from rehearsalhub.schemas.song import SongRead
+
+
+class RehearsalSessionRead(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+    id: uuid.UUID
+    band_id: uuid.UUID
+    date: datetime
+    nc_folder_path: str | None = None
+    label: str | None = None
+    notes: str | None = None
+    created_at: datetime
+    recording_count: int = 0
+
+
+class RehearsalSessionDetail(RehearsalSessionRead):
+    songs: list[SongRead] = []
+
+
+class RehearsalSessionUpdate(BaseModel):
+    label: str | None = None
+    notes: str | None = None