feat(api): session router — list, detail, patch endpoints

GET /bands/{id}/sessions — list with recording counts, newest first
GET /bands/{id}/sessions/{sid} — session detail with flat song list
PATCH /bands/{id}/sessions/{sid} — admin: update label/notes

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

api/src/rehearsalhub/routers/__init__.py

@@ -3,6 +3,7 @@ from rehearsalhub.routers.auth import router as auth_router
 from rehearsalhub.routers.bands import router as bands_router
 from rehearsalhub.routers.internal import router as internal_router
 from rehearsalhub.routers.members import router as members_router
+from rehearsalhub.routers.sessions import router as sessions_router
 from rehearsalhub.routers.songs import router as songs_router
 from rehearsalhub.routers.versions import router as versions_router
 from rehearsalhub.routers.ws import router as ws_router
@@ -12,6 +13,7 @@ __all__ = [
     "bands_router",
     "internal_router",
     "members_router",
+    "sessions_router",
     "songs_router",
     "versions_router",
     "annotations_router",