feat(#168): API couples de garde parent (liste + couple courant).

Co-authored-by: Cursor <cursoragent@cursor.com>

database/migrations/2026_parent_placement_garde_courant.sql

@@ -0,0 +1,10 @@
+-- Ticket #168 — Couple de garde courant (préférence parent)
+-- Idempotent : safe à rejouer.
+
+ALTER TABLE parents
+  ADD COLUMN IF NOT EXISTS id_placement_garde_courant UUID
+    REFERENCES enfants_assistantes_maternelles(id) ON DELETE SET NULL;
+
+CREATE INDEX IF NOT EXISTS idx_parents_placement_garde_courant
+  ON parents(id_placement_garde_courant)
+  WHERE id_placement_garde_courant IS NOT NULL;