feat(P1): FTS5 virtual table for email search (replaces LIKE scan)

Creates an external-content FTS5 virtual table `email_fts` backed by
the `emails` table. Three triggers (AI/AU/AD) keep the index in sync
automatically. Schema bumped to v26; backfill runs on upgrade.

`searchEmailsGlobal` now issues a ranked FTS5 JOIN instead of
`LIKE '%word%'` scans, making search O(log n) via the inverted index.
Words are expanded to prefix terms (`word*`) so partial matches still
work.

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

lib/data/db/database.dart

@@ -269,10 +269,47 @@ class AppDatabase extends _$AppDatabase {
   AppDatabase([QueryExecutor? executor]) : super(executor ?? _openConnection());
 
   @override
-  int get schemaVersion => 25;
+  int get schemaVersion => 26;
+
+  Future<void> _createEmailFts() async {
+    await customStatement('''
+      CREATE VIRTUAL TABLE IF NOT EXISTS email_fts USING fts5(
+        subject, preview, from_json,
+        content='emails',
+        content_rowid='rowid'
+      )
+    ''');
+    await customStatement('''
+      CREATE TRIGGER IF NOT EXISTS email_fts_ai
+      AFTER INSERT ON emails BEGIN
+        INSERT INTO email_fts(rowid, subject, preview, from_json)
+        VALUES (new.rowid, new.subject, new.preview, new.from_json);
+      END
+    ''');
+    await customStatement('''
+      CREATE TRIGGER IF NOT EXISTS email_fts_au
+      AFTER UPDATE OF subject, preview, from_json ON emails BEGIN
+        INSERT INTO email_fts(email_fts, rowid, subject, preview, from_json)
+        VALUES ('delete', old.rowid, old.subject, old.preview, old.from_json);
+        INSERT INTO email_fts(rowid, subject, preview, from_json)
+        VALUES (new.rowid, new.subject, new.preview, new.from_json);
+      END
+    ''');
+    await customStatement('''
+      CREATE TRIGGER IF NOT EXISTS email_fts_ad
+      AFTER DELETE ON emails BEGIN
+        INSERT INTO email_fts(email_fts, rowid, subject, preview, from_json)
+        VALUES ('delete', old.rowid, old.subject, old.preview, old.from_json);
+      END
+    ''');
+  }
 
   @override
   MigrationStrategy get migration => MigrationStrategy(
+        onCreate: (m) async {
+          await m.createAll();
+          await _createEmailFts();
+        },
         onUpgrade: (m, from, to) async {
           // NOTE: m.createTable(T) creates the LATEST version of table T.
           // If you later add a column C to T in version X, you must guard
@@ -447,6 +484,14 @@ class AppDatabase extends _$AppDatabase {
               ),
             );
           }
+          if (from < 26) {
+            await _createEmailFts();
+            // Backfill FTS index from existing rows.
+            await customStatement('''
+              INSERT INTO email_fts(rowid, subject, preview, from_json)
+              SELECT rowid, subject, preview, from_json FROM emails
+            ''');
+          }
         },
       );
 }

lib/data/repositories/email_repository_impl.dart

@@ -2470,28 +2470,39 @@ class EmailRepositoryImpl implements EmailRepository {
     String? accountId,
     String query,
   ) async {
+    final ftsQuery = _toFtsQuery(query);
+    if (ftsQuery.isEmpty) return [];
+
+    final sql = accountId != null
+        ? 'SELECT e.* FROM email_fts f JOIN emails e ON e.rowid = f.rowid'
+            ' WHERE email_fts MATCH ? AND e.account_id = ? ORDER BY rank LIMIT 50'
+        : 'SELECT e.* FROM email_fts f JOIN emails e ON e.rowid = f.rowid'
+            ' WHERE email_fts MATCH ? ORDER BY rank LIMIT 50';
+    final variables = accountId != null
+        ? [Variable<String>(ftsQuery), Variable<String>(accountId)]
+        : [Variable<String>(ftsQuery)];
+
+    final queryRows = await _db
+        .customSelect(sql, variables: variables, readsFrom: {_db.emails}).get();
+    final emailRows = await Future.wait(
+      queryRows.map((r) => _db.emails.mapFromRow(r)),
+    );
+    return emailRows.map(_toModel).toList();
+  }
+
+  /// Converts a user query string into an FTS5 match expression.
+  /// Each whitespace-separated word becomes a prefix term (word*) so that
+  /// partial words still match. Special FTS5 characters are stripped.
+  static String _toFtsQuery(String query) {
     final words = query
-        .toLowerCase()
+        .trim()
         .split(RegExp(r'\s+'))
         .where((w) => w.isNotEmpty)
+        .map((w) => w.replaceAll(RegExp(r'[^\w]'), ''))
+        .where((w) => w.isNotEmpty)
         .toList();
-    final rows = await (_db.select(_db.emails)
-          ..where((t) {
-            Expression<bool> condition = const Constant(true);
-            if (accountId != null) {
-              condition = t.accountId.equals(accountId);
-            }
-            for (final word in words) {
-              final pattern = '%$word%';
-              condition = condition &
-                  (t.subject.like(pattern) | t.preview.like(pattern));
-            }
-            return condition;
-          })
-          ..orderBy([(t) => OrderingTerm.desc(t.receivedAt)])
-          ..limit(50))
-        .get();
-    return rows.map(_toModel).toList();
+    if (words.isEmpty) return '';
+    return words.map((w) => '$w*').join(' ');
   }
 
   @override

test/unit/migration_test.dart

@@ -14,7 +14,7 @@ void main() {
   group('Migration', () {
     test('schemaVersion matches expected value', () async {
       final db = AppDatabase(NativeDatabase.memory());
-      expect(db.schemaVersion, 25);
+      expect(db.schemaVersion, 26);
       await db.close();
     });
 
@@ -158,6 +158,19 @@ void main() {
         ]),
       );
 
+      // v26: FTS5 virtual table and triggers exist.
+      final allTriggers = await db
+          .customSelect("SELECT name FROM sqlite_master WHERE type='trigger'")
+          .get();
+      final triggerNames =
+          allTriggers.map((r) => r.read<String>('name')).toSet();
+      expect(
+        triggerNames,
+        containsAll(['email_fts_ai', 'email_fts_au', 'email_fts_ad']),
+      );
+      // Verify FTS table was created and is queryable.
+      await db.customSelect('SELECT count(*) FROM email_fts').get();
+
       await db.close();
       if (dbFile.existsSync()) dbFile.deleteSync();
     });
@@ -276,11 +289,23 @@ void main() {
       expect(indexNames, contains('mailboxes_account_id'));
       expect(indexNames, contains('threads_latest_date'));
 
+      // v26: FTS5 virtual table and triggers.
+      final allTriggers = await db
+          .customSelect("SELECT name FROM sqlite_master WHERE type='trigger'")
+          .get();
+      final triggerNames =
+          allTriggers.map((r) => r.read<String>('name')).toSet();
+      expect(
+        triggerNames,
+        containsAll(['email_fts_ai', 'email_fts_au', 'email_fts_ad']),
+      );
+      await db.customSelect('SELECT count(*) FROM email_fts').get();
+
       await db.close();
       if (dbFile.existsSync()) dbFile.deleteSync();
     });
 
-    test('fresh install creates all tables at schemaVersion 25', () async {
+    test('fresh install creates all tables at schemaVersion 26', () async {
       final db = AppDatabase(NativeDatabase.memory());
       await db.select(db.accounts).get();