feat: persistent SQLite storage for bookmarks + positions keyed by content_hash

- new bookish/services/database.py: SQLite with WAL mode, tables for
  bookmarks and reading_positions, keyed by content_hash (SHA256)
- new bookish/routes/api.py: REST endpoints for CRUD bookmarks
  and save/load reading position
- upload endpoint returns content_hash (SHA256 of file)
- reader route computes SHA256 and passes BOOKISH_HASH to template
- reader.js: bookmarks + positions use API server as primary storage
  with localStorage fallback for offline/resilience
- position saves debounced (1.5s) to avoid hammering server
- .gitignore: added bookish.db

.gitignore

@@ -5,3 +5,4 @@ uploads/
 converted/
 .venv.bak/
 .sisyphus/
+bookish.db