feat(materials): Фаза 2 — коллекции (папки), поиск и фильтры

- Миграция 061: material_collections + student_materials.collection_id (ON DELETE SET NULL) + tags.
- API: CRUD коллекций (/api/materials/collections), GET /materials отдаёт {materials, collections}
  со счётчиками; PATCH /materials/:id принимает collection_id/tags. Хелперы в js/api.js.
- /my-materials: бар папок (Все/папки/Без папки/+папка) с фильтром, поиск по тексту, фильтр по типу,
  перенос материала в папку (select на карточке), создание/переименование/удаление папок.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/src/db/migrations/061_material_collections.sql

@@ -0,0 +1,21 @@
+-- ═══════════════════════════════════════════════════════════════
+-- 061: Collections (folders) + tags for «Мои материалы»
+--
+-- A user groups saved materials into named collections (folders). A material
+-- belongs to at most one collection (collection_id, SET NULL when the folder
+-- is deleted — the material stays, just becomes "uncategorised"). Optional
+-- free-text tags for quick filtering.
+-- ═══════════════════════════════════════════════════════════════
+
+CREATE TABLE material_collections (
+  id          INTEGER PRIMARY KEY AUTOINCREMENT,
+  user_id     INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+  name        TEXT NOT NULL,
+  color       TEXT,
+  sort_order  INTEGER NOT NULL DEFAULT 0,
+  created_at  TEXT NOT NULL DEFAULT (datetime('now'))
+);
+CREATE INDEX idx_material_collections_user ON material_collections(user_id, sort_order, id);
+
+ALTER TABLE student_materials ADD COLUMN collection_id INTEGER REFERENCES material_collections(id) ON DELETE SET NULL;
+ALTER TABLE student_materials ADD COLUMN tags TEXT;