feat(journeyitem): V74 — partial unique index + note-length CHECK as atomic backstops

The append dedup guard was check-then-insert: two concurrent appends of the
same document both pass exists() and both insert. The partial unique index
on (geschichte_id, document_id) WHERE document_id IS NOT NULL closes the
race; append saveAndFlush-es and maps the DataIntegrityViolationException to
the same 409 JOURNEY_DOCUMENT_ALREADY_ADDED as the friendly pre-check. The
CHECK on note length pins the 2000-char contract in the schema, mirroring
chk_text_length on transcription_blocks.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/architecture/db/db-orm.puml

@@ -377,9 +377,10 @@ package "Supporting" {
     geschichte_id : UUID <<FK>>
     document_id : UUID <<FK>>
     position : INTEGER NOT NULL CHECK (position > 0)
-    note : TEXT
+    note : TEXT CHECK (length <= 2000)
     ==
     UNIQUE (geschichte_id, position) DEFERRABLE INITIALLY DEFERRED
+    UNIQUE (geschichte_id, document_id) WHERE document_id IS NOT NULL
   }
 }