Sync reader draw ink to server for cross-device persistence

Previously reader ink was localStorage-only so it never followed the
user to a different browser or device.

- New reader_ink table (user_id, book_abbrev, chapter, strokes JSON)
- GET /api/reader/ink  — loads all pages for the signed-in user
- PUT /api/reader/ink/:book/:chapter — upserts one page
- On startup, client fetches server ink and merges it over localStorage
  (server wins so the most-recently saved version always wins)
- updateReaderPageInk debounces a PUT call 1 s after the last stroke,
  so rapid Apple Pencil strokes don't flood the server

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

server/db.js

@@ -69,6 +69,18 @@ export function initDb() {
   }
   db.exec('CREATE UNIQUE INDEX IF NOT EXISTS idx_projects_share_token ON projects(share_token) WHERE share_token IS NOT NULL');
 
+  // Reader ink — one row per user + book + chapter, stored as JSON
+  db.exec(`
+    CREATE TABLE IF NOT EXISTS reader_ink (
+      user_id     TEXT NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+      book_abbrev TEXT NOT NULL,
+      chapter     INTEGER NOT NULL,
+      strokes     TEXT NOT NULL,
+      updated_at  INTEGER NOT NULL,
+      PRIMARY KEY (user_id, book_abbrev, chapter)
+    );
+  `);
+
   console.log(`SQLite database ready at ${DB_PATH}`);
 }
 
@@ -146,6 +158,30 @@ export function setBackupCodeHashes(userId, backupCodeHashes) {
   db.prepare('UPDATE users SET backup_codes = ? WHERE id = ?').run(JSON.stringify(backupCodeHashes), userId);
 }
 
+// ---------------------------------------------------------------------------
+// Reader ink — cross-device sync for draw-mode annotations in the reader
+// ---------------------------------------------------------------------------
+
+/** Returns all saved reader ink pages for a user as { "BOOK_CH": strokes[] }. */
+export function getAllReaderInk(userId) {
+  const rows = db.prepare(
+    'SELECT book_abbrev AS book, chapter, strokes FROM reader_ink WHERE user_id = ?'
+  ).all(userId);
+  return Object.fromEntries(
+    rows.map((r) => [`${r.book}_${r.chapter}`, JSON.parse(r.strokes)])
+  );
+}
+
+/** Upserts the ink strokes for one reader page. */
+export function setReaderInkPage(userId, bookAbbrev, chapter, strokes) {
+  db.prepare(`
+    INSERT INTO reader_ink (user_id, book_abbrev, chapter, strokes, updated_at)
+    VALUES (?, ?, ?, ?, ?)
+    ON CONFLICT(user_id, book_abbrev, chapter)
+    DO UPDATE SET strokes = excluded.strokes, updated_at = excluded.updated_at
+  `).run(userId, bookAbbrev, Number(chapter), JSON.stringify(strokes), Date.now());
+}
+
 /**
  * Assigns any pre-existing, unowned projects (from before multi-user support)
  * to the given user. Intended to run once, right after the first account is created.