feat(db): add context_text column to reading_progress

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)

internal/database/models.go

@@ -380,6 +380,7 @@ type ReadingProgress struct {
 	Percentage        pgtype.Float8      `db:"percentage" json:"percentage"`
 	CharacterOffset   pgtype.Int8        `db:"character_offset" json:"character_offset"`
 	Epubcfi           pgtype.Text        `db:"epubcfi" json:"epubcfi"`
+	ContextText       pgtype.Text        `db:"context_text" json:"context_text"`
 	Chapter           pgtype.Int4        `db:"chapter" json:"chapter"`
 	ChapterProgress   pgtype.Float8      `db:"chapter_progress" json:"chapter_progress"`
 	ViewportX         pgtype.Float8      `db:"viewport_x" json:"viewport_x"`