chore(memoro): remove old NestJS backends (Phase 8+9)

Delete apps/memoro/apps/backend/ (NestJS) and apps/memoro/apps/audio-backend/
(NestJS) — all functionality has been ported to the new Hono/Bun servers
(apps/server/ and apps/audio-server/).

Also clean up root and memoro package.json scripts to remove references
to the old @memoro/backend and @memoro/audio-backend packages.

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

packages/manadeck-database/src/schema/studySessions.ts

@@ -1,39 +0,0 @@
-import { pgTable, uuid, text, integer, timestamp, index, pgEnum } from 'drizzle-orm/pg-core';
-import { relations } from 'drizzle-orm';
-import { decks } from './decks.js';
-
-// Study mode enum
-export const studyModeEnum = pgEnum('study_mode', ['all', 'new', 'review', 'favorites', 'random']);
-
-export const studySessions = pgTable(
-	'study_sessions',
-	{
-		id: uuid('id').primaryKey().defaultRandom(),
-		deckId: uuid('deck_id')
-			.notNull()
-			.references(() => decks.id, { onDelete: 'cascade' }),
-		userId: text('user_id').notNull(),
-		mode: studyModeEnum('mode').notNull(),
-		totalCards: integer('total_cards').notNull().default(0),
-		completedCards: integer('completed_cards').notNull().default(0),
-		correctCards: integer('correct_cards').notNull().default(0),
-		startedAt: timestamp('started_at', { withTimezone: true }).defaultNow().notNull(),
-		completedAt: timestamp('completed_at', { withTimezone: true }),
-		timeSpentSeconds: integer('time_spent_seconds').default(0).notNull(),
-	},
-	(table) => [
-		index('idx_study_sessions_user_id').on(table.userId),
-		index('idx_study_sessions_deck_id').on(table.deckId),
-		index('idx_study_sessions_started_at').on(table.startedAt),
-	]
-);
-
-export const studySessionsRelations = relations(studySessions, ({ one }) => ({
-	deck: one(decks, {
-		fields: [studySessions.deckId],
-		references: [decks.id],
-	}),
-}));
-
-export type StudySession = typeof studySessions.$inferSelect;
-export type NewStudySession = typeof studySessions.$inferInsert;