feat(stammbaum): add V54 migration for family network

Adds persons.family_member flag and person_relationships table with
ON DELETE CASCADE on both FKs, no_self_rel check, unique_rel composite,
indexes on both person columns, and partial unique index for symmetric
SIBLING_OF pairs (LEAST/GREATEST trick).

Refs #358.

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

backend/src/main/resources/db/migration/V54__add_family_network.sql

@@ -0,0 +1,30 @@
+-- Family network: marks a Person as a tree node and stores typed relationships
+-- between two persons. The tree page (/stammbaum) only shows persons with
+-- family_member = TRUE. Symmetric types (SPOUSE_OF, SIBLING_OF) are stored once;
+-- the partial unique index keeps SIBLING_OF pairs from being duplicated in the
+-- reverse direction.
+
+ALTER TABLE persons
+    ADD COLUMN family_member BOOLEAN NOT NULL DEFAULT FALSE;
+
+CREATE TABLE person_relationships (
+    id                 UUID         PRIMARY KEY DEFAULT gen_random_uuid(),
+    person_id          UUID         NOT NULL REFERENCES persons(id) ON DELETE CASCADE,
+    related_person_id  UUID         NOT NULL REFERENCES persons(id) ON DELETE CASCADE,
+    relation_type      VARCHAR(30)  NOT NULL,
+    from_year          INTEGER,
+    to_year            INTEGER,
+    notes              VARCHAR(2000),
+    created_at         TIMESTAMPTZ  NOT NULL DEFAULT now(),
+    CONSTRAINT no_self_rel CHECK (person_id <> related_person_id),
+    CONSTRAINT unique_rel  UNIQUE (person_id, related_person_id, relation_type)
+);
+
+CREATE INDEX idx_person_rel_person_id         ON person_relationships(person_id);
+CREATE INDEX idx_person_rel_related_person_id ON person_relationships(related_person_id);
+
+-- Symmetric SIBLING_OF: enforce only one row per unordered pair.
+CREATE UNIQUE INDEX unique_sibling_pair ON person_relationships (
+    LEAST(person_id, related_person_id),
+    GREATEST(person_id, related_person_id)
+) WHERE relation_type = 'SIBLING_OF';