fix(migration): add CHECK constraint on text length (defense in depth)

V18: text column now has CHECK (length(text) <= 10000) to enforce
the 10,000 character limit at the database level, complementing
the application-level enforcement in TranscriptionService.sanitizeText().

Fixes @Nora: "DB constraint catches anything the application misses"

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

backend/src/main/resources/db/migration/V18__add_transcription_blocks.sql

@@ -2,7 +2,7 @@ CREATE TABLE transcription_blocks (
     id            UUID             PRIMARY KEY DEFAULT gen_random_uuid(),
     annotation_id UUID             NOT NULL REFERENCES document_annotations(id) ON DELETE RESTRICT,
     document_id   UUID             NOT NULL REFERENCES documents(id) ON DELETE CASCADE,
-    text          TEXT             NOT NULL DEFAULT '',
+    text          TEXT             NOT NULL DEFAULT '' CHECK (length(text) <= 10000),
     label         VARCHAR(200),
     sort_order    INTEGER          NOT NULL DEFAULT 0,
     version       INTEGER          NOT NULL DEFAULT 0,