Add queued email notifications for collaboration events

apps/server/internal/database/migrations/000016_email_queue.up.sql

@@ -0,0 +1,18 @@
+CREATE TABLE IF NOT EXISTS email_queue (
+    id TEXT PRIMARY KEY,
+    notification_id TEXT,
+    to_address TEXT NOT NULL,
+    subject TEXT NOT NULL,
+    body TEXT NOT NULL,
+    status TEXT NOT NULL DEFAULT 'pending' CHECK(status IN ('pending', 'sent', 'failed', 'skipped')),
+    attempts INTEGER NOT NULL DEFAULT 0,
+    max_attempts INTEGER NOT NULL DEFAULT 3,
+    last_error TEXT,
+    next_attempt_at TEXT NOT NULL,
+    sent_at TEXT,
+    created_at TEXT NOT NULL,
+    FOREIGN KEY(notification_id) REFERENCES notifications(id) ON DELETE SET NULL
+);
+
+CREATE INDEX IF NOT EXISTS idx_email_queue_status ON email_queue(status, next_attempt_at);
+CREATE INDEX IF NOT EXISTS idx_email_queue_notification ON email_queue(notification_id);