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/utils/followNotifications.js

@@ -0,0 +1,50 @@
+const Database = require('../Database')
+const SocketAuthority = require('../SocketAuthority')
+const Logger = require('../Logger')
+
+/**
+ * Notify users following a series that a new book was added.
+ * Called after a new library item is created by the scanner.
+ *
+ * @param {import('../models/LibraryItem')} libraryItem - the newly created expanded library item
+ */
+async function notifyFollowersOfNewBook(libraryItem) {
+  if (libraryItem.mediaType !== 'book') return
+  if (!libraryItem.media?.series?.length) return
+
+  const seriesIds = libraryItem.media.series.map((s) => s.id)
+  if (!seriesIds.length) return
+
+  const follows = await Database.userSeriesFollowModel.findAll({
+    where: { seriesId: seriesIds },
+    attributes: ['userId', 'seriesId']
+  })
+
+  if (!follows.length) return
+
+  // Group by userId to send one event per user
+  const userSeriesMap = {}
+  for (const follow of follows) {
+    if (!userSeriesMap[follow.userId]) {
+      userSeriesMap[follow.userId] = []
+    }
+    userSeriesMap[follow.userId].push(follow.seriesId)
+  }
+
+  const libraryItemJson = libraryItem.toOldJSONExpanded()
+
+  for (const [userId, followedSeriesIds] of Object.entries(userSeriesMap)) {
+    const matchedSeries = libraryItem.media.series
+      .filter((s) => followedSeriesIds.includes(s.id))
+      .map((s) => ({ id: s.id, name: s.name }))
+
+    SocketAuthority.clientEmitter(userId, 'followed_series_book_added', {
+      libraryItem: libraryItemJson,
+      series: matchedSeries
+    })
+
+    Logger.debug(`[FollowNotifications] Notified user ${userId} of new book in followed series: ${matchedSeries.map((s) => s.name).join(', ')}`)
+  }
+}
+
+module.exports = { notifyFollowersOfNewBook }