add reading settings sync

migrations/0009_add_user_reading_prefs.down.sql

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+ALTER TABLE users DROP COLUMN IF EXISTS reading_prefs;

migrations/0009_add_user_reading_prefs.up.sql

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+-- Adds per-user reading preferences (theme palette, font family, font
+-- size, justification toggle) so the prefs roam across devices once the
+-- user signs in.
+--
+-- Stored as JSONB rather than a column-per-field — the shape is owned
+-- by the frontend (components/reading-settings.tsx), changes
+-- occasionally (new palettes added every now and then), and we never
+-- query individual keys server-side. JSONB keeps the schema stable as
+-- the shape evolves.
+--
+-- Default `'{}'::jsonb` so existing rows are not nulled and the
+-- frontend can treat the server response as "merge into local defaults"
+-- without an additional null check.
+ALTER TABLE users
+    ADD COLUMN IF NOT EXISTS reading_prefs JSONB NOT NULL DEFAULT '{}'::jsonb;