feat(backend): add indexes to SleepSession start_time and end_time

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/alembic/versions/a3282e5bef40_add_indexes_to_sleep_session.py

@@ -0,0 +1,34 @@
+"""add indexes to sleep_session
+
+Revision ID: a3282e5bef40
+Revises: 7a424a341b1d
+Create Date: 2026-06-12 22:55:33.711961
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'a3282e5bef40'
+down_revision: Union[str, Sequence[str], None] = '7a424a341b1d'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.create_index(op.f('ix_sleep_session_end_time'), 'sleep_session', ['end_time'], unique=False)
+    op.create_index(op.f('ix_sleep_session_start_time'), 'sleep_session', ['start_time'], unique=False)
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_index(op.f('ix_sleep_session_start_time'), table_name='sleep_session')
+    op.drop_index(op.f('ix_sleep_session_end_time'), table_name='sleep_session')
+    # ### end Alembic commands ###

backend/app/models.py

@@ -7,7 +7,7 @@ class SleepSession(SQLModel, table=True):
     __tablename__ = "sleep_session"
 
     id: Optional[int] = Field(default=None, primary_key=True)
-    start_time: datetime = Field(nullable=False)
-    end_time: Optional[datetime] = Field(default=None, nullable=True)
+    start_time: datetime = Field(index=True, nullable=False)
+    end_time: Optional[datetime] = Field(default=None, nullable=True, index=True)
     created_at: datetime = Field(default_factory=lambda: datetime.now(timezone.utc))
     updated_at: datetime = Field(default_factory=lambda: datetime.now(timezone.utc))