feat(calendar): implement local calendar invitations with email notifications

- Added functionality to create, accept, decline, and delete local calendar invitations.
- Integrated email notifications for calendar event invitations and deletions.
- Updated database schema to support local invitations and outbound event tracking.
- Enhanced the calendar UI to display pending invitations and allow users to manage them.
- Localized new strings for invitations in English and German.

server/config/database.js

@@ -558,6 +558,65 @@ export async function initDatabase() {
     `);
   }
 
+  // Track outbound calendar event federation sends for deletion propagation
+  if (isPostgres) {
+    await db.exec(`
+      CREATE TABLE IF NOT EXISTS calendar_event_outbound (
+        id SERIAL PRIMARY KEY,
+        event_uid TEXT NOT NULL,
+        remote_domain TEXT NOT NULL,
+        created_at TIMESTAMP DEFAULT NOW(),
+        UNIQUE(event_uid, remote_domain)
+      );
+      CREATE INDEX IF NOT EXISTS idx_cal_out_event_uid ON calendar_event_outbound(event_uid);
+    `);
+  } else {
+    await db.exec(`
+      CREATE TABLE IF NOT EXISTS calendar_event_outbound (
+        id INTEGER PRIMARY KEY AUTOINCREMENT,
+        event_uid TEXT NOT NULL,
+        remote_domain TEXT NOT NULL,
+        created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+        UNIQUE(event_uid, remote_domain)
+      );
+      CREATE INDEX IF NOT EXISTS idx_cal_out_event_uid ON calendar_event_outbound(event_uid);
+    `);
+  }
+
+  // Local calendar event invitations (share-with-acceptance flow)
+  if (isPostgres) {
+    await db.exec(`
+      CREATE TABLE IF NOT EXISTS calendar_local_invitations (
+        id SERIAL PRIMARY KEY,
+        event_id INTEGER NOT NULL REFERENCES calendar_events(id) ON DELETE CASCADE,
+        from_user_id INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+        to_user_id INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+        status TEXT DEFAULT 'pending' CHECK(status IN ('pending','accepted','declined')),
+        created_at TIMESTAMP DEFAULT NOW(),
+        UNIQUE(event_id, to_user_id)
+      );
+      CREATE INDEX IF NOT EXISTS idx_cal_local_inv_to_user ON calendar_local_invitations(to_user_id);
+      CREATE INDEX IF NOT EXISTS idx_cal_local_inv_event ON calendar_local_invitations(event_id);
+    `);
+  } else {
+    await db.exec(`
+      CREATE TABLE IF NOT EXISTS calendar_local_invitations (
+        id INTEGER PRIMARY KEY AUTOINCREMENT,
+        event_id INTEGER NOT NULL,
+        from_user_id INTEGER NOT NULL,
+        to_user_id INTEGER NOT NULL,
+        status TEXT DEFAULT 'pending' CHECK(status IN ('pending','accepted','declined')),
+        created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+        UNIQUE(event_id, to_user_id),
+        FOREIGN KEY (event_id) REFERENCES calendar_events(id) ON DELETE CASCADE,
+        FOREIGN KEY (from_user_id) REFERENCES users(id) ON DELETE CASCADE,
+        FOREIGN KEY (to_user_id) REFERENCES users(id) ON DELETE CASCADE
+      );
+      CREATE INDEX IF NOT EXISTS idx_cal_local_inv_to_user ON calendar_local_invitations(to_user_id);
+      CREATE INDEX IF NOT EXISTS idx_cal_local_inv_event ON calendar_local_invitations(event_id);
+    `);
+  }
+
   // ── Default admin (only on very first start) ────────────────────────────
   const adminAlreadySeeded = await db.get("SELECT value FROM settings WHERE key = 'admin_seeded'");
   if (!adminAlreadySeeded) {