feat(geschichte): add entity, status enum, and V58 schema migration

Geschichte holds family memory stories (issue #381). Body is unbounded TEXT
(Tiptap HTML, no length limit). Two join tables link a story to historical
Persons and Documents. A partial index speeds the public index query
(status='PUBLISHED' ORDER BY published_at DESC) and reverse-lookup indexes
support the ?personId and ?documentId filters.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/src/main/resources/db/migration/V58__add_geschichten.sql

@@ -0,0 +1,34 @@
+-- Geschichten: blog-like family memory stories linked to persons and documents (issue #381).
+-- BLOG_WRITE permission gates authoring; DRAFT stories are never returned to readers.
+
+CREATE TABLE geschichten (
+    id              UUID PRIMARY KEY,
+    title           VARCHAR(255) NOT NULL,
+    body            TEXT,
+    status          VARCHAR(32)  NOT NULL,
+    author_id       UUID         REFERENCES app_users (id) ON DELETE SET NULL,
+    created_at      TIMESTAMP    NOT NULL,
+    updated_at      TIMESTAMP    NOT NULL,
+    published_at    TIMESTAMP
+);
+
+CREATE TABLE geschichten_persons (
+    geschichte_id   UUID NOT NULL REFERENCES geschichten (id) ON DELETE CASCADE,
+    person_id       UUID NOT NULL REFERENCES persons      (id) ON DELETE CASCADE,
+    PRIMARY KEY (geschichte_id, person_id)
+);
+
+CREATE TABLE geschichten_documents (
+    geschichte_id   UUID NOT NULL REFERENCES geschichten (id) ON DELETE CASCADE,
+    document_id     UUID NOT NULL REFERENCES documents   (id) ON DELETE CASCADE,
+    PRIMARY KEY (geschichte_id, document_id)
+);
+
+-- Index page query: WHERE status = 'PUBLISHED' ORDER BY published_at DESC.
+CREATE INDEX idx_geschichten_published
+    ON geschichten (published_at DESC)
+    WHERE status = 'PUBLISHED';
+
+-- Reverse-lookup indexes for the ?personId / ?documentId filters.
+CREATE INDEX idx_geschichten_persons_person     ON geschichten_persons   (person_id);
+CREATE INDEX idx_geschichten_documents_document ON geschichten_documents (document_id);