Add backend support for users to follow series

Introduces a UserSeriesFollow model with a dedicated join table so users
can follow/unfollow series and receive socket notifications when new
books are added to followed series. The user JSON response now includes a
seriesFollowing array, and three new API endpoints are available under
/api/me/follows/.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/migrations/v2.34.0-create-user-series-follows.js

@@ -0,0 +1,110 @@
+/**
+ * @typedef MigrationContext
+ * @property {import('sequelize').QueryInterface} queryInterface - a suquelize QueryInterface object.
+ * @property {import('../Logger')} logger - a Logger object.
+ *
+ * @typedef MigrationOptions
+ * @property {MigrationContext} context - an object containing the migration context.
+ */
+
+const migrationVersion = '2.34.0'
+const migrationName = `${migrationVersion}-create-user-series-follows`
+const loggerPrefix = `[${migrationVersion} migration]`
+
+/**
+ * This upward migration creates the userSeriesFollows table for tracking
+ * which users follow which series.
+ *
+ * @param {MigrationOptions} options - an object containing the migration context.
+ * @returns {Promise<void>} - A promise that resolves when the migration is complete.
+ */
+async function up({ context: { queryInterface, logger } }) {
+  logger.info(`${loggerPrefix} UPGRADE BEGIN: ${migrationName}`)
+
+  if (await queryInterface.tableExists('userSeriesFollows')) {
+    logger.info(`${loggerPrefix} table "userSeriesFollows" already exists`)
+  } else {
+    logger.info(`${loggerPrefix} creating table "userSeriesFollows"`)
+    const DataTypes = queryInterface.sequelize.Sequelize.DataTypes
+    await queryInterface.createTable('userSeriesFollows', {
+      id: {
+        type: DataTypes.UUID,
+        defaultValue: DataTypes.UUIDV4,
+        primaryKey: true
+      },
+      createdAt: {
+        type: DataTypes.DATE,
+        allowNull: false
+      },
+      userId: {
+        type: DataTypes.UUID,
+        references: {
+          model: {
+            tableName: 'users'
+          },
+          key: 'id'
+        },
+        allowNull: false,
+        onDelete: 'CASCADE'
+      },
+      seriesId: {
+        type: DataTypes.UUID,
+        references: {
+          model: {
+            tableName: 'series'
+          },
+          key: 'id'
+        },
+        allowNull: false,
+        onDelete: 'CASCADE'
+      }
+    })
+    logger.info(`${loggerPrefix} created table "userSeriesFollows"`)
+
+    // Index for "get all series a user follows" (most frequent query)
+    await queryInterface.addIndex('userSeriesFollows', {
+      name: 'user_series_follows_userId',
+      fields: ['userId']
+    })
+    logger.info(`${loggerPrefix} added index on userId`)
+
+    // Unique constraint to prevent duplicate follows
+    await queryInterface.addIndex('userSeriesFollows', {
+      name: 'user_series_follows_unique',
+      fields: ['userId', 'seriesId'],
+      unique: true
+    })
+    logger.info(`${loggerPrefix} added unique index on (userId, seriesId)`)
+
+    // Index for "find all followers of series X" (for notifications)
+    await queryInterface.addIndex('userSeriesFollows', {
+      name: 'user_series_follows_seriesId',
+      fields: ['seriesId']
+    })
+    logger.info(`${loggerPrefix} added index on seriesId`)
+  }
+
+  logger.info(`${loggerPrefix} UPGRADE END: ${migrationName}`)
+}
+
+/**
+ * This downward migration script removes the userSeriesFollows table.
+ *
+ * @param {MigrationOptions} options - an object containing the migration context.
+ * @returns {Promise<void>} - A promise that resolves when the migration is complete.
+ */
+async function down({ context: { queryInterface, logger } }) {
+  logger.info(`${loggerPrefix} DOWNGRADE BEGIN: ${migrationName}`)
+
+  if (await queryInterface.tableExists('userSeriesFollows')) {
+    logger.info(`${loggerPrefix} dropping table "userSeriesFollows"`)
+    await queryInterface.dropTable('userSeriesFollows')
+    logger.info(`${loggerPrefix} dropped table "userSeriesFollows"`)
+  } else {
+    logger.info(`${loggerPrefix} table "userSeriesFollows" does not exist`)
+  }
+
+  logger.info(`${loggerPrefix} DOWNGRADE END: ${migrationName}`)
+}
+
+module.exports = { up, down }