feat: add in-app notification system with real-time delivery

Introduces a full in-app notification system with three types (simple,
boolean with server-side callbacks, navigate), three scopes (user, trip,
admin), fan-out persistence per recipient, and real-time push via
WebSocket. Includes a notification bell in the navbar, dropdown, dedicated
/notifications page, and a dev-only admin tab for testing all notification
variants.

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

server/src/db/schema.ts

@@ -394,6 +394,30 @@ function createTables(db: Database.Database): void {
       ip TEXT
     );
     CREATE INDEX IF NOT EXISTS idx_audit_log_created ON audit_log(created_at DESC);
+
+    CREATE TABLE IF NOT EXISTS notifications (
+      id INTEGER PRIMARY KEY AUTOINCREMENT,
+      type TEXT NOT NULL CHECK(type IN ('simple', 'boolean', 'navigate')),
+      scope TEXT NOT NULL CHECK(scope IN ('trip', 'user', 'admin')),
+      target INTEGER NOT NULL,
+      sender_id INTEGER REFERENCES users(id) ON DELETE SET NULL,
+      recipient_id INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+      title_key TEXT NOT NULL,
+      title_params TEXT DEFAULT '{}',
+      text_key TEXT NOT NULL,
+      text_params TEXT DEFAULT '{}',
+      positive_text_key TEXT,
+      negative_text_key TEXT,
+      positive_callback TEXT,
+      negative_callback TEXT,
+      response TEXT CHECK(response IN ('positive', 'negative')),
+      navigate_text_key TEXT,
+      navigate_target TEXT,
+      is_read INTEGER DEFAULT 0,
+      created_at DATETIME DEFAULT CURRENT_TIMESTAMP
+    );
+    CREATE INDEX IF NOT EXISTS idx_notifications_recipient ON notifications(recipient_id, is_read, created_at DESC);
+    CREATE INDEX IF NOT EXISTS idx_notifications_recipient_created ON notifications(recipient_id, created_at DESC);
   `);
 }