feat(db): add schema for bookmarks and meta tables

schema.sql

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+-- Bookmarks store. Rows accumulate so the feed keeps tweets even after they
+-- scroll out of X's 100-item bookmarks API window.
+CREATE TABLE IF NOT EXISTS bookmarks (
+  id         TEXT PRIMARY KEY,   -- tweet id
+  text       TEXT NOT NULL,
+  author_id  TEXT,
+  username   TEXT,
+  name       TEXT,
+  created_at TEXT,               -- ISO 8601 from X
+  media_urls TEXT,               -- JSON array of media URLs
+  url        TEXT,               -- permalink
+  fetched_at INTEGER NOT NULL    -- epoch ms when first stored
+);
+CREATE INDEX IF NOT EXISTS idx_bookmarks_created ON bookmarks(created_at DESC);
+
+-- Tiny key/value table for OAuth tokens, user id, PKCE verifiers, and last_sync.
+CREATE TABLE IF NOT EXISTS meta (
+  key   TEXT PRIMARY KEY,
+  value TEXT NOT NULL
+);