feat(search): extend hasText to match sender/receiver/tag names, add hasTagPartial

- hasText now JOINs sender (LEFT JOIN) and uses EXISTS subqueries for
  receivers and tags to avoid duplicate rows
- hasTagPartial added for live debounced tag text filter (ILIKE partial match)

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

backend/src/test/java/org/raddatz/familienarchiv/repository/DocumentSpecificationsTest.java

@@ -250,6 +250,62 @@ class DocumentSpecificationsTest {
         assertThat(result).isEmpty();
     }
 
+    @Test
+    void hasText_findsByPartialSenderLastName() {
+        List<Document> result = documentRepository.findAll(Specification.where(hasText("üller")));
+        assertThat(result).extracting(Document::getTitle)
+                .containsExactlyInAnyOrder("Alter Brief", "Neuerer Brief");
+    }
+
+    @Test
+    void hasText_findsByPartialReceiverLastName() {
+        List<Document> result = documentRepository.findAll(Specification.where(hasText("schmid")));
+        assertThat(result).extracting(Document::getTitle).containsExactly("Alter Brief");
+    }
+
+    @Test
+    void hasText_findsByPartialTagName() {
+        List<Document> result = documentRepository.findAll(Specification.where(hasText("amili")));
+        assertThat(result).extracting(Document::getTitle)
+                .containsExactlyInAnyOrder("Alter Brief", "Familienfoto");
+    }
+
+    @Test
+    void hasText_doesNotProduceDuplicatesForDocumentWithMultipleReceivers() {
+        Person receiver2 = personRepository.save(Person.builder().firstName("Karl").lastName("Schmidt").build());
+        briefEarly.setReceivers(new java.util.HashSet<>(Set.of(receiver, receiver2)));
+        documentRepository.save(briefEarly);
+
+        List<Document> result = documentRepository.findAll(Specification.where(hasText("schmid")));
+        assertThat(result).hasSize(1);
+    }
+
+    // ─── hasTagPartial ────────────────────────────────────────────────────────
+
+    @Test
+    void hasTagPartial_returnsAllDocuments_whenTextIsNull() {
+        List<Document> result = documentRepository.findAll(Specification.where(hasTagPartial(null)));
+        assertThat(result).hasSize(3);
+    }
+
+    @Test
+    void hasTagPartial_findsByPartialTagName() {
+        List<Document> result = documentRepository.findAll(Specification.where(hasTagPartial("amili")));
+        assertThat(result).extracting(Document::getTitle).containsExactly("Alter Brief");
+    }
+
+    @Test
+    void hasTagPartial_isCaseInsensitive() {
+        List<Document> result = documentRepository.findAll(Specification.where(hasTagPartial("URLAUB")));
+        assertThat(result).extracting(Document::getTitle).containsExactly("Neuerer Brief");
+    }
+
+    @Test
+    void hasTagPartial_returnsEmpty_whenNoTagMatches() {
+        List<Document> result = documentRepository.findAll(Specification.where(hasTagPartial("xyz")));
+        assertThat(result).isEmpty();
+    }
+
     // ─── hasStatus ────────────────────────────────────────────────────────────
 
     @Test