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/song.py

@@ -15,6 +15,7 @@ class SongUpdate(BaseModel):
     title: str | None = None
     status: Literal["jam", "wip", "arranged", "recorded", "released"] | None = None
     notes: str | None = None
+    tags: list[str] | None = None
     global_key: str | None = None
     global_bpm: float | None = None
 
@@ -23,8 +24,10 @@ class SongRead(BaseModel):
     model_config = ConfigDict(from_attributes=True)
     id: uuid.UUID
     band_id: uuid.UUID
+    session_id: uuid.UUID | None = None
     title: str
     status: str
+    tags: list[str] = []
     global_key: str | None = None
     global_bpm: float | None = None
     notes: str | None = None