fix(db): add indexes on documents.sender_id and document_comments.author_id

Flyway V62 adds idx_documents_sender_id and idx_comments_author_id to speed up
FK-driven queries on the persons page and briefwechsel view. Closes #470.

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

backend/src/main/resources/db/migration/V62__index_fk_columns.sql

@@ -0,0 +1,8 @@
+-- Speeds up "documents by sender" queries used on /persons/[id] Korrespondenz-Überblick (#306),
+-- /briefwechsel, and bulk-edit flows.
+CREATE INDEX IF NOT EXISTS idx_documents_sender_id
+    ON documents(sender_id);
+
+-- Speeds up "comments by author" queries on admin user detail and (future) contributor profile.
+CREATE INDEX IF NOT EXISTS idx_comments_author_id
+    ON document_comments(author_id);

backend/src/test/java/org/raddatz/familienarchiv/MigrationIntegrationTest.java

@@ -399,6 +399,24 @@ class MigrationIntegrationTest {
               AND dc.annotation_id IS NOT NULL
             """;
 
+    // ─── V62: indexes on FK columns ──────────────────────────────────────────
+
+    @Test
+    void v62_idx_documents_sender_id_exists() {
+        Integer count = jdbc.queryForObject(
+                "SELECT COUNT(*) FROM pg_catalog.pg_indexes WHERE tablename = 'documents' AND indexname = 'idx_documents_sender_id'",
+                Integer.class);
+        assertThat(count).isEqualTo(1);
+    }
+
+    @Test
+    void v62_idx_comments_author_id_exists() {
+        Integer count = jdbc.queryForObject(
+                "SELECT COUNT(*) FROM pg_catalog.pg_indexes WHERE tablename = 'document_comments' AND indexname = 'idx_comments_author_id'",
+                Integer.class);
+        assertThat(count).isEqualTo(1);
+    }
+
     // ─── V63+V64: group_permissions dedup + primary key ──────────────────────
 
     @Test