Stores surrounding text (~100 chars) at the reader's current position.
Used as fallback for CFI resolution when converting between epubcfi
and CREngine XPointer formats.
Updates:
- schema.sql: add context_text TEXT column, update stored procedure
- queries.sql: add context_text to GetUniversalProgress and
UpdateUniversalProgress queries
- Regenerate sqlc Go code (models.go, querier.go, queries.sql.go)