feat: split initial social schema migration chain

replace monolithic migration with ordered scripts, include profiles/sessions in migration, and verify full downgrade/upgrade cycle for clean Supabase bootstrap

backend/alembic/versions/20260226_0005_hardening_indexes.py

@@ -0,0 +1,29 @@
+"""initial schema part 5: hardening and partial indexes
+
+Revision ID: 202602260005
+Revises: 202602260004
+Create Date: 2026-02-26 20:14:00
+"""
+
+from typing import Sequence, Union
+
+from alembic import op
+
+revision: str = "202602260005"
+down_revision: Union[str, Sequence[str], None] = "202602260004"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.execute(
+        "CREATE INDEX ix_inbox_messages_pending_recent ON inbox_messages (recipient_id, created_at DESC) WHERE status = 'pending'"
+    )
+    op.execute(
+        "CREATE INDEX ix_todos_pending_due ON todos (owner_id, due_at) WHERE status = 'pending'"
+    )
+
+
+def downgrade() -> None:
+    op.execute("DROP INDEX IF EXISTS ix_todos_pending_due")
+    op.execute("DROP INDEX IF EXISTS ix_inbox_messages_pending_recent")