feat(documents): add ScriptType enum and script_type column

- ScriptType enum: UNKNOWN, TYPEWRITER, HANDWRITING_LATIN, HANDWRITING_KURRENT
- V24 migration adds script_type VARCHAR(30) NOT NULL DEFAULT 'UNKNOWN'
- Document entity: scriptType field with @Builder.Default UNKNOWN
- DocumentUpdateDTO: optional scriptType field
- DocumentService: wires scriptType through update method

Refs #226

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/resources/db/migration/V24__add_script_type_to_documents.sql

@@ -0,0 +1 @@
+ALTER TABLE documents ADD COLUMN script_type VARCHAR(30) NOT NULL DEFAULT 'UNKNOWN';