feat(server): an entry's text keys alone find it in running text

A dictionary file may name an entry's `textKeys`, its own forms: those
alone fill `dictionary_key`, so a German word is found in a subtitle by
its forms, never by its English meanings, which would match English
words in German text; a search still finds it by them. Files without
them (JMdict's) fill it with every key without a space, as before.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_server/test/integration/dictionary/dictionary_test.dart

@@ -672,6 +672,54 @@ void main() {
         );
       });
 
+      test('withKeys finds an entry that names its text keys by them alone: '
+          'a German word by its forms, never by its English meanings, which '
+          'would match English words in German text', () async {
+        await loadDictionaryFile(
+          session,
+          write(
+            [
+              {
+                ...entry(
+                  'Haus/noun',
+                  'Haus',
+                  rank: 5584,
+                  keys: ['haus', 'häuser', 'house', 'house, building'],
+                ),
+                'textKeys': ['haus', 'häuser', 'häusern'],
+              },
+            ],
+            name: 'wiktionary-de-en.jsonl',
+            head: {
+              ...header(source: 'wiktionary-de'),
+              'language': 'de',
+            },
+          ),
+        );
+
+        final found = await endpoints.dictionary.withKeys(
+          signedIn,
+          'wiktionary-de',
+          'en',
+          ['Häusern', 'house', 'haus'],
+        );
+
+        expect({for (final hit in found) hit.key}, {'häusern', 'haus'});
+        // A search still finds it by its meaning.
+        expect(
+          [
+            for (final hit in await endpoints.dictionary.search(
+              signedIn,
+              'wiktionary-de',
+              'en',
+              'house',
+            ))
+              hit.headword,
+          ],
+          ['Haus'],
+        );
+      });
+
       test('withKeys finds the keys loaded with their entries, and no more '
           'once a load replaced them; a dictionary loaded before keys had '
           'rows loads again', () async {