fix(database): cache unavailable FTS5 capability

lib/services/history_database.dart

@@ -74,7 +74,9 @@ class HistoryDatabase {
   static final HistoryDatabase instance = HistoryDatabase._init();
   static final sqlite.SingleFlightInitializer<Database> _database =
       sqlite.SingleFlightInitializer<Database>();
-  bool _searchFtsAvailable = false;
+  // null means this database connection has not attempted FTS setup yet.
+  // false is a completed capability/setup result and must not be retried.
+  bool? _searchFtsAvailable;
 
   HistoryDatabase._init();
 
@@ -89,14 +91,12 @@ class HistoryDatabase {
       // onCreate normally initializes this derived index. Retry once after
       // opening an existing database in case an earlier setup was
       // interrupted; unsupported SQLite builds remain on the LIKE fallback.
-      if (!_searchFtsAvailable) {
-        _searchFtsAvailable = await _createSearchFts(db);
-      }
+      _searchFtsAvailable ??= await _createSearchFts(db);
       return db;
     });
   }
 
-  bool get searchFtsAvailable => _searchFtsAvailable;
+  bool get searchFtsAvailable => _searchFtsAvailable ?? false;
 
   Future<void> _createDB(Database db, int version) async {
     _log.i('Creating database schema v$version');
@@ -1079,6 +1079,7 @@ class HistoryDatabase {
     final db = await database;
     await db.close();
     _database.reset();
+    _searchFtsAvailable = null;
   }
 
   Future<void> updateFilePath(

lib/services/library_database.dart

@@ -26,7 +26,9 @@ class LibraryDatabase {
   static final sqlite.SingleFlightInitializer<Database> _database =
       sqlite.SingleFlightInitializer<Database>();
   bool _historyAttached = false;
-  bool _searchFtsAvailable = false;
+  // null means this database connection has not attempted FTS setup yet.
+  // false is a completed capability/setup result and must not be retried.
+  bool? _searchFtsAvailable;
 
   LibraryDatabase._init();
 
@@ -41,14 +43,12 @@ class LibraryDatabase {
       // onCreate normally initializes this derived index. Retry once after
       // opening an existing database in case an earlier setup was
       // interrupted; unsupported SQLite builds remain on the LIKE fallback.
-      if (!_searchFtsAvailable) {
-        _searchFtsAvailable = await _createSearchFts(db);
-      }
+      _searchFtsAvailable ??= await _createSearchFts(db);
       return db;
     });
   }
 
-  bool get searchFtsAvailable => _searchFtsAvailable;
+  bool get searchFtsAvailable => _searchFtsAvailable ?? false;
 
   Future<void> _ensureHistoryAttached(Database db) async {
     if (_historyAttached) return;
@@ -1659,6 +1659,7 @@ class LibraryDatabase {
     await db.close();
     _database.reset();
     _historyAttached = false;
+    _searchFtsAvailable = null;
   }
 
   Future<Map<String, int>> getFileModTimes({String? sourceId}) async {

lib/services/sqlite_helpers.dart

@@ -6,6 +6,9 @@ import 'package:sqflite/sqflite.dart';
 
 final _log = AppLogger('AppSqlite');
 
+Future<bool>? _trigramFts5Capability;
+const _trigramFts5ProbeTable = 'spotiflac_trigram_fts5_probe';
+
 /// Caches an asynchronously-created value while also coalescing concurrent
 /// callers onto the same in-flight initialization.
 class SingleFlightInitializer<T extends Object> {
@@ -108,6 +111,57 @@ String? ftsPhraseSearchQuery(String value) {
   return '"${value.replaceAll('"', '""')}"';
 }
 
+/// Whether [error] means the current SQLite runtime cannot provide the
+/// FTS5/trigram combination used by the search indexes.
+bool isTrigramFts5UnavailableError(Object error) {
+  final message = error.toString().toLowerCase();
+  return message.contains('no such module: fts5') ||
+      message.contains('no such tokenizer: trigram') ||
+      message.contains('unknown tokenizer: trigram');
+}
+
+/// Probes FTS5 + trigram once for the current process.
+///
+/// Android's sqflite implementation uses the platform SQLite runtime, so
+/// compile-time modules can vary by device. A real temporary virtual table is
+/// more reliable than PRAGMA compile_options because it also verifies that the
+/// trigram tokenizer is registered. All app databases use the same sqflite
+/// runtime, so subsequent history/library initialization can reuse the result.
+Future<bool> _supportsTrigramFts5(DatabaseExecutor db) {
+  return _trigramFts5Capability ??= _probeTrigramFts5(db);
+}
+
+Future<bool> _probeTrigramFts5(DatabaseExecutor db) async {
+  try {
+    await db.execute('''
+      CREATE VIRTUAL TABLE temp.$_trigramFts5ProbeTable USING fts5(
+        search_text,
+        tokenize='trigram'
+      )
+    ''');
+    return true;
+  } catch (error) {
+    if (isTrigramFts5UnavailableError(error)) {
+      _log.i(
+        'Trigram FTS5 is unavailable in this SQLite runtime; '
+        'search will use the LIKE fallback',
+      );
+    } else {
+      _log.w(
+        'Could not probe trigram FTS5; search will use the LIKE fallback: '
+        '$error',
+      );
+    }
+    return false;
+  } finally {
+    try {
+      await db.execute('DROP TABLE IF EXISTS temp.$_trigramFts5ProbeTable');
+    } catch (error) {
+      _log.d('Could not remove the temporary FTS5 probe table: $error');
+    }
+  }
+}
+
 /// Creates an external-content FTS5 index that preserves substring search
 /// semantics through SQLite's trigram tokenizer.
 ///
@@ -122,6 +176,8 @@ Future<bool> createTrigramFtsIndex(
   required String contentTable,
   required String triggerPrefix,
 }) async {
+  if (!await _supportsTrigramFts5(db)) return false;
+
   try {
     final existingIndex = await db.rawQuery(
       "SELECT 1 FROM sqlite_master WHERE type = 'table' AND name = ? LIMIT 1",

test/sqlite_helpers_test.dart

@@ -103,5 +103,35 @@ void main() {
         expect(ftsPhraseSearchQuery('a\u0000b'), isNull);
       },
     );
+
+    test('recognizes missing FTS5 and trigram capability errors', () {
+      expect(
+        isTrigramFts5UnavailableError(
+          Exception('DatabaseException(no such module: fts5)'),
+        ),
+        isTrue,
+      );
+      expect(
+        isTrigramFts5UnavailableError(
+          Exception('DatabaseException(no such tokenizer: trigram)'),
+        ),
+        isTrue,
+      );
+      expect(
+        isTrigramFts5UnavailableError(Exception('database is locked')),
+        isFalse,
+      );
+    });
+
+    test('probes the runtime with a temporary trigram FTS5 table', () {
+      final source = File(
+        'lib/services/sqlite_helpers.dart',
+      ).readAsStringSync();
+
+      expect(source, contains('CREATE VIRTUAL TABLE temp.'));
+      expect(source, contains("tokenize='trigram'"));
+      expect(source, contains('DROP TABLE IF EXISTS temp.'));
+      expect(source, contains('_trigramFts5Capability ??='));
+    });
   });
 }